Are you using thumb slugs or just holes drilled into the ball?  Regardless, it could be different materials giving you different friction or it could indeed be too much reverse in the thumbhole.  You could try having a ball drilled with less reverse and see how that works or you could just adjust the fit of the thumbholes by using more/less tape as required.

The rest below is all from the following link:

http://www.bowl4fun.com/ron/btm04_files/btm4.htm

First off, a proper fitting ball should feel comfortable in your hand when the ball is sitting still and in the full motion of a swing and release. There should be no pain felt in the fingers, thumb or any part of the hand that can be associated with the bowling ball. The ball should stay on your thumb with very little or no squeezing until the ball reaches the very bottom of the swing (point of release), where the pseudo centrifugal force combines with the weight of the ball to pull the ball off the thumb and onto the fingers. That last sentence is a very important one; notice it should take pseudo centrifugal force “added” to the weight of the ball to pull ball off of the thumb. The weight of a hanging ball alone should not be able to pull the ball off of your thumb if you keep your wrist straight and your grip relaxed.
 
The care and feeding of a thumbhole
 
The thumbhole of a bowling ball requires just as much care and feeding as any pet I have ever had. If you look into your thumbhole and all you see is an empty hole, then the odds of you becoming a great bowler just got a lot slimmer.
Thumbholes require a steady diet of tape (there are many types) and or Ron C’s Magic Carpet. There are several reasons an empty thumbhole spells trouble:
1.      If your thumbhole is empty and you “think” you can throw the ball pretty well, the thumbhole is likely too loose on your thumb. When the thumbhole is too loose, you MUST squeeze the ball tightly to keep from dropping it; this destroys any hope of performing a good armswing or release.
 
2.      If your thumbhole is empty but yet very snug fitting to the thumb, then you are in trouble also. You see, an empty thumbhole is full of air until you stick your thumb in there, which forces most of the air out of the hole as your thumb goes in. 
When you couple a snug hole with a nice soft thumb you have basically just built a syringe, those scary things the doctor uses to give you shots. Syringes are great for moving fluids in and out but they don’t make very good thumbholes.

With an empty, but snug thumbhole, everything looks and feels just fine if you just set your ball on a table and pull your thumb in and out. The difference shows up when you actually throw the ball and the thumb needs to exit very quickly; the low pressure area in the bottom of the thumbhole will try to keep the thumb from coming out due to suction. Sometimes this suction is so subtle you can’t feel it and other times you can, even making a popping sound. One clue this may be happening to a bowler is the use of a slick powder on the thumb like EZ Slide. More about venting the thumbhole for a quick release later. (Note: a popping sound from the fingers is fine.)

 
 
3.      Most bowlers’ thumbs change size at least a little from day to day or even game to game. I could never stress enough how important it is for the thumbhole to perfectly fit the thumb at all times. In order to keep the thumbhole fitting perfectly, you must change its size using bowlers tape, Ron C’s Magic Carpet or some other means. If you have something like that in your thumbhole then your thumbhole will not be empty.
 
Taping the thumbhole
 
Taping the thumbhole needs to accomplish two things:
 
 #1: Create an airway so the thumb can move in and out without changing the air pressure in the thumbhole.
Many bowlers know to use tape to take up space in the hole, but most don’t know that it should also function as an air pressure equalizer. It takes at least three pieces of white tape layered on top of each other to produce two tiny airways on each side of the layered tape. This is about the bare minimum of airflow necessary to get a quick clean release from the thumb if the hole is snug, so keep this in mind when sizing your thumbhole.
You need to maintain at least three pieces of tape layered in the ball even when the thumb is swollen. It is ok to add as many pieces of tape as necessary to tighten the hole when the thumb shrinks, but I don’t think it is a good idea to have more than five pieces of tape in the front of the hole. Too many pieces of tape in the front will start to change the span and thumb pitch. If you don’t have room in your thumbhole for at least three pieces of tape after your thumb swells you can ether open the hole up or have your driller dill you a tiny vent hole.
            A vent hole is a tiny hole that is placed beside the thumb and exits near the bottom of the thumbhole. Vent holes used to be popular years ago and are covered by USBC rules, but they have disappeared over time; modern thumb slugs have vent holes built into them, but they often get clogged after being installed into the ball.
Ron C’s Magic Carpet is designed to flow air all around and through the product so only one piece is used.
#2 Keep the thumbhole the exact right fit.
Tape is added or removed to keep the thumbhole the exact right fit for the thumb as the thumb changes size. Very few thumbs (if any) are always the same size so something must be added or subtracted from the thumbhole to keep the fit perfect at all times. Tape can be added to the front of the thumbhole or the back or both. Ron C’s Magic Carpet always goes in the back of the thumbhole so white tape can be added to the front of the thumbhole if needed. I think that all tape should be placed at least one-forth of an inch below the top of the hole and each added piece should be placed one-sixteenth of an inch below the previous piece for easy removal.

Keep in mind that the “front” of the thumbhole means where the pad of the thumb comes in contact with the thumbhole, so it is usually a little offset from the very top of the hole. The “back” of the thumbhole refers to where the knuckle of the thumb comes in contact with the thumbhole and again this is usually offset from the center of the bottom of the hole.
 
How tight should the thumbhole be?
 
In my opinion, the thumbhole should be tight enough that you don’t have to squeeze the ball during the swing but not so tight that you have to really force your thumb into the ball.
From my experience, there are a lot more bowlers that have their thumbholes too loose than too tight or just right. I can’t tell you how many times I will tell a student that I need to add a couple pieces of tape to their ball only to hear them say “Are you sure? My thumbhole is pretty tight.” After adding five or six pieces of tape and noticing a marked improvement in the armswing and release, the student starts to understand that their thumbhole was too loose. They were not aware they were using the Kung Fu Death Grip because if feels natural and normal, but it wrecks havoc with the swing and release.
 
Finding the right snugness is really a simple exercise. Start with the thumbhole a little too loose and add tape until you feel the ball hang a little on the thumb after a few full speed releases, then take a piece out. Keep in mind that with each added piece of tape, you can squeeze a little less so you may want to throw several shots and allow your hand to relax a little before you decide to take a piece out. Don’t stop adding tape just because you feel like you have added a lot. If at the end of the process you find that you had to add seven or more pieces of tape, you may want to consider dropping your thumbhole size one notch the next time you drill a ball.
Many times during this process bowlers will discover that by the time they add enough tape to the hole to reduce squeezing a significant amount, they can hardly get their thumb into the hole any longer because it is so tight. If your thumbhole is so tight that you have to force your thumb into the hole, then you most likely need to move your thumb pitch more forward. With the thumb pitch moved more forward, the hole will not have to be as tight to stay on your thumb.

Wow!  I have been using tape for many years and learned a lot of what is in the article the hard way, but hadn't seen everything covered in one place before.  Just various bits and pieces as they pertained to a particular question or post.

So I saw a lot of useful information (thank you), but three or four points jumped out at me for now;

1)Tightness-- Have always preferred a very tight thumbhole over the years, and usually had the hole drilled to accomodate 3-4 pieces of tape in the winter, so I could take out 2 or 3 of them in the summer when the temperatue and humidity (and 9 months of frequent bowling) would grow my thumb. Always wanted one piece, even then, cause it gave me some friction and because I felt I had a margin for error in case my thumb swelled even more due to extreme conditions.

Am so anal about this that I often go back to the driller and ask that the hole be opened up or increased one drill-bit size, even if it fits perfectly, if I am not able to get the correct number of pieces of tape into the ball for that time of year (4 in the winter, 1 in the summer, etc..). 

This year, though, I had a conflict in that while recovering from my knee surgery I was much more paranoid about the thumb being too tight as I did not want to hang up and be pulled forward on my recovering knee.  So THIS year, I had tended to err on the side of less tape versus more.  It was supposed to be a temporary change, but had forgotten and had only recently ----about a week or two ago---realized this and started adding more tape again.....with good results.

2) Forward Pitch-- Currently all of my balls are (or are SUPPOSED to be) zero pitch, but did experiment with forward pitch a few years ago. The results were good and I liked not having to be so dependent on the number of pieces of tape and felt confident in the release and decided to have my entire arsenal converted. Problem was that mid-way through the conversion the driller made a mistake and drilled reverse pitch in two of the balls and I was so disillusioned afterward (was dropping eveyrthing ----always) that I abandoned the project (and that driller) and wen t back to zero pitch in everything. My thinking went something like it was easier to go with more conventional drilling so future drillers couldn't as easily screw up.  ---I know.... silly.  But was looking for safe, reliable, easily duplicated drilling.

3) Turning of the wrist-- I have always been guilty of turning my wrist too early in an attempt to get more revs on the ball (am most definitely NOT a cranker).  Of course doing that not only doesn't add revs, but it tends to reduce them and increaethe liklihood of dropping the ball cause your hand is on top of the ball while swinging it. Part of the reason this was much more of a problem this year is again related to the knee surgery.  When I was in mid-season form in previous years, I was probably doing SOMETHING that counter-balanced that bad habit, still enabling me to throw a decent ball.  This year----until a week ago---has been a disaster.

I have been working on NOT turning my wrist until the last possible moment.  Probably only a delay of a second or so, but definitely helping.

4) "Modern thumb slugs have vent holes built into them"-- I do use thumb slugs in all my equipment and I did not know this about the vent holes, but then I'm not sure how far back we're talking when the word "modern" is used.

Some of the balls are older, but oldest slugs date back to 2004 or 2006. Would there be a substantial difference in texture or design between slugs from that era and now?

And yet, when I last attempted to have a driller identify the problem a month ago and suggested that maybe there was a difference in texture between the slugs in my older equipment and the most recently drilled balls, he didn't seem to think so.

SOME of my circa 2004-2006 slugs are vinyl, whereas all the current stuff is urethane.  And to bring this full-circle (from my original posted question), blowing in the thumbhole of the urethane slugged ball SEEMED to make it feel more like the vinyl ones.  Not tighter, not necessarily any obvious increase in friction, but was as if there was a little bit of air pressure in the hole helping my thumb stay in the ball a fraction fo a second longer.

Was very comforting, cause it wasn't the kind of tug or pull that would be strong enough to pull me down the lane or onto my knee. Just a very slight bit of pressure that was missing before I blew into it. 

And I didn;t need to blow every frame.  Just once in awhile ---every 3-4 frames---and increased my confidence and that helped straighten out other chinks in my delivery too (or at least I THOUGHT it was doing that, which is maybe as good as uf it actually did).

But with all these fixes/corrections, and being able to overcome whatever is different about the drillings on he 3 suspect balls, I still believe these is some slight difference between them and the rest of my balls, and yet....  not sure how to help the driller determine what that is.  Has to be something very small, but I don't believe it's non-existent.

And even if I find out what it is (maybe is the material of the thumbslugs) I may decide is not worth the hassle of fixing. But I'd like to have the option instead of just accepting that there is no difference----even though my gut is telling me there is.

 You posted while I was replying, and I saw something that I want to make a point of:

 

SOME of my circa 2004-2006 slugs are vinyl, whereas all the current stuff is urethane.  And to bring this full-circle (from my original posted question), blowing in the thumbhole of the urethane slugged ball SEEMED to make it feel more like the vinyl ones.  Not tighter, not necessarily any obvious increase in friction, but was as if there was a little bit of air pressure in the hole helping my thumb stay in the ball a fraction fo a second longer.

 There is a DEFINITE feel difference between vinyl and urethane slugs, with the urethane always being "slicker" that the vinyl ones.

 I once tried urethane slugs, and found the same condition you have, that I needed a little extra "something" to get the same eel from them, even though the hole specs were the same.

Forgot to comment on the balls possibly being undersized.

I doubt that's the case ith 2 of the 3 balls in question, but one of them is definitely undersized, and although I've had it for 7-8 years (and was bought brand new at a brick and mortar pro-shop) just recently discovered this.  The pro-shop operator took a ring and showed that it passed through whereas another ball (regulation size0 did not.

So I can see where that might be a factor, but this ball did not have a dropping problem until it was recently plugged and re-drilled. And specs have been checked and re-checked.  But I'll bet they didn't check the material of the slug which is currently urerthane and most likely was vinyl previously.

Always thought that hole was just a means of lining things up, as it's usually just about in the middle of where my thumb goes (for purposes of putting tape in the right spot).  Is so tiny I never realized it had a function as a vent.

And I am surprised about the size of the ball not mattering [to the point of being illegal].  Especially since the pro-shop driller who said this never tries to push a new ball on me.

I see what you mean ("...At time of manufacture, all balls must meet the following
technical specifications when tested....), but the particular ball in question does not have that many game son it, so if it's smaller than specs now it probably always was.

But legalities aside, am curious for a ball that is slightly smaller in diameter/circumference, what would the results be as far as ball reaction, etc.,?  Even if very slight, would it make sharper turns or burn-up faster or otherwise manifest any different behavior just by being a hair smaller than the norm?