I'm a left handed bowler.  The ball always hang on the outside of my thumb (right side of thumb on left hand).  I have a big callus there.  I keep trying to open up the side of the thumb hole but it seems to always catch there.  Does anybody know of anything else to try that will help?

what I mean by "hangs" is that it feels like the entire weight of the ball sits on the outside of my left thumb at the knuckle.  After every time I bowl, it always feel sore at the spot.