try taking a bevel sander and lightly sanding out a oval where your thumb needs to release.  I do this on all my balls to clear easily.  if you do not have a beveler just make a tool like I do.  take a large sized mark a lot, apply double tape (carpet tape) and then stick some sandpaper on it and use it to sand remove where needed.  this tool works really good for me
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"I'd quit this game if I thought anyone would give a damn!"

"If Fishing is a Sport, well then you're looking at an athlete!"

talk to your driller then look into some small pitch adjustments. that or try shortening your span a lil unless you have pretty large hands a 5" is quite long

Well i put my thumb in the ball lay my fingers over the holes and the first crease is centered over the holes. Is that right ? Im gonna try more reverse in a test ball here soon. see what that does.