Yeah, I use thumb slugs in my stuff.  I wonder if I just have too much reverse pitch?
--------------------
You wanna bet me how much?  Let's play!

Yes, those Master protective patches work GREAT!
BUT
pinbuster is probably right: your thumb hole is not right.

Since you're one of those 220 average bowlers, has this not happened before? has your hand changed recently? lose some weight maybe?
My fingers/thumb/hand shrink something terrible in the winter, not so much from the cold but from the reduced humidity. Some years I even change my thumb inserts to one size smaller. In fact thinking of doing that right now.
--------------------
"Just because you can do something does not mean you should do it."

I think my thumb hole is to big also, but I am not going to get it plugged and drilled smaller, I think I am going to try and get some of that bowlers tape that you just slap on the inside. That way, when the warmer seasons do come around, if my thumb swells a lil from the humidity and what not, I can just pull the tape out, give it a good cleaning as to get the sticky junk out, and be GOOD TO GO!
--------------------
15 years and still going strong! 15 years old that is! DA DA DAAAAAA!! UP UP AND AWAY! TO THE BAT CAVE ROBIN!