A thumb insert is really just to make every thumb hole feel the same. When you have several bowling balls, different coverstock, filler, and core materials can feel differently in the thumb hole, the insert simply gives you the same feel from ball to ball. They also make it easier to change the size of your thumb hole. If one is too large or too small, the existing slug can be removed and a new one put in without having to plug and re-drill the hole.
If the ball doesn't fit properly, then you driller didn't do it right. You should not have to put a lot of effort into holding onto the ball unless you have a very exaggerated backswing.
If it's just a little to big, try adding a piece or two of tape to the inside of the hole. Your pro shop can hook you up with the tape. It should be free if he did a poor job measuring your thumb; the alternative is to plug and re-drill the hole, also on his dime, and a few pieces of tape will cost him pennies.
If it's more than a little too big (I've had that happen to me, combine a slightly swollen thumb with a little too much exuberance with the bevel sander), tape won't do an adequate job. You might ask about putting in a slug that fits. He can bore out the existing hole and put in a properly-sized slug. He will probably charge for that; adding inserts is nearly always an extra charge on top of drilling. Had you gotten that in the first place, you'd've paid for it anyway.
SH
