Hey nebowler, Welcome to the Track forum.
Many of the current balls today which have a lower rg have a lighter material used to fill around the core which can cause this to happen. Another reason is if your driller's bit is getting a little dull also.
Normally, not to worry so long as it's not irritating your hand. Actually, have your driller put on a new sanding disk (with one old behind it too) and sand against the part that moved if you don't want to use a thumb insert/slug. Hope this helps explain some...
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Rick Leong - Ten Pins Pro Shop
Track Intl. - Amateur/Pro Shop Staff
Vise Inserts Staff
Trackbowling.com