don't use the thumb but a friend has really experimented with forward pitches in his thumb and he found out that he needed to offset the forward in the thumb with reverse in the fingers.

How many games did you bowl with the 1/8th forward before adding in bevel? Myself, I've done many experiments with changing forward pitch, and sometimes the original impression isn't best to go off of.  If didn't bowl much with it, it may be best to just give it some time.

Adding bevel will make come out easier. A really sharp bevel will cause hang. What may have been doing originally was compensating for too little forward by using a very sharp bevel. It may be that more even more forward pitch is needed, with the added bevel. But between bevel, pitch, and then span changes there are a lot of tweaks that can be made.

The degree of bevel can have a profound effect on feel and is rarely talked about. Sharp bevel causes thumb to hang, but it's sort of like a fake way of keeping thumb in ball longer. More forward pitch secures the base of thumb in hole. Doing it with bevel is similar, but it's more like getting a sharp edge to stick, and it will be hard to have consistent feel from session to session or different seasons/time day doing it that way.

Keep in mind what many people who start to add forward pitch learn is the longer they use it, the more forward pitch they feel comfortable with.

Myself have used 0 forward up to where am now at 1/2 forward. I've tried every pitch in between. I'm not a PSO, just some guy who has done a lot of experimenting. I have found for myself that span changes to compensate for pitch is very, very overrated. Yet, if you add in 1/4 forward, maybe need to alter span slightly, but any needed span change from only 1/8 forward pitch would by my guess be minuscule.

In your case reducing span will cause ball to come out faster. Since you want the ball to stay on thumb longer, then if a hypothetical span change is called for, it would be more span.

Adding reverse in the fingers, in this case, would cause ball to fall off of fingers, so then it's slipping out of thumb, and coming off fingers faster - not good. Yes, it is possible for adding forward pitch to cause my tension in fingers (not my experience at all, but others do find it happens).

So conclusion from random guy on Internet who has tried dozens of these changes -

If it was slipping originally probably needed more forward pitch and you had been compensating by using very sharp bevel. Once added forward pitch had trouble matching bevel to the pitch. Added in some bevel and started to slip again.

What I would do if me. Add more forward pitch and keep moderate bevel.

for anyone else reading, what causes need for more forward pitch:
-short span
-short thumb
-dry/slippery thumb

Is your ball swing and approach in perfect sync??  Are you holding the ball correctly? The easy, but sometimes wrong approach is to fool with grip.

John, I do that a bit, yes. It's something I'm working on. For purposes of this discussion though, we can assume that all releases are done as cleanly as I am currently capable of doing, which is admittedly imperfect.

To continue JohnP's reply. Probably the most common problem bowlers have is holding onto the ball.  There are  myriad reasons why it is difficult to hold onto the ball and release consistently.  Until they are addressed, attacking the grip is of no use.  A certain amount of "control" can be attained by quick fixes, but if you don't have all the variables kind of accounted for, you're not fixing anything.  If you and your PSO are fixated on just your grip, good luck.

It is impossible to properly hold onto the ball without a correct grip. Trying to fix other things when the grip is wrong is beyond pointless.

You can not out bowl a poor grip.

It is impossible.

It's also not difficult to find guys who drop the ball every show and just know they need forward pitch.

I had like 10 guy years ago insist that I must have reverse pitch.
Then quit bowling for years.

When got back into it, somehow ended up driving like 3 hours to see Mike Luongo, he took one glance at my hand and told me I needed at least 1/4 forward pitch.

In this case, if one thing consistently happens with zero pitch and another with 1/8 forward, then it's a grip issue.

Can't repeat enough - you can not out-bowl a poor grip.
Nor can you out-coach a poor grip.
Obviously as someone improves and learns what works, they can modify and perfect their grip, but you can't get anywhere if the grip does not allow you to hold onto the ball.

Btw there is NO perfect grip or angle for gripping...and the bowler should not be gripping the ball...it is supported by the wrist and having it properly fit to your hand that keeps it on the bowlers hand...gravity & momentum is what releases it...not the bowler needing to 'get' it off their hand
Two things effect the creation of the swing...the amount of muscle in the arm & grip which causes its redirection...and the amount of pull effecting the timing
You can have a 'perfect' grip and terrible timing and you can have a terrible grip that creates the timing but everyone is individual primarily due to strength
A steeper swing plane may require more 'help' from the gripping angles in supporting the ball and a flatter swing plane may require less