I am a lefty, speed around 15.5, probably average revs for a lefty.  I usually roll across 8/9 at the arrows out to about the 5/6 board.  I've averaged around 220 for a few years now, but recently I have been tracking over my thumb after I set the ball down.  This only happens when I bowl w/o a wrist support.  IF I put on my wrist device (storm xtra roll wrist support), I don't track over my thumb.  Am I just coming around the ball too soon w/o the brace on?  Am I thumbing down?  I really want to get away from the wrist support but the sound of my ball rolling over my thumb on the first couple revs is driving me nuts.

Any ideas on what I could be doing wrong?

I was a full roller for over 40 years.  Through some experimenting with pitches and using the Storm Xtra Roll I got my track down under the thumb. However, if I roll the ball without the wrist support I will full roll it.  In my case it is because I cannot keep my wrist straight without the support.  Just my thoughts.