i think the material could be a big problem...i switched over to urethane a few months ago - it never changes size compared to teh vinyl i used to have...i have lessened teh strain on my thumb when going thru my backswing, and as a result my thumb doesnt swell as much and my release is much more comfortable. the vinyl always used to change sizes. one option may be to try a custon thumb slug...get it made a little loose, put some tape in, and then work with that...my driller can make them out of a urethane substance, and they should never change size. and like revless said, if u are wearing a wrist guard, that could definitely be your problem. and try some ez slide in between shots(wipe off before preparing for next shot) to keep your thumb smooth and dry. that helps me during the summer when my thumb grows.
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Erik Olsen - e13ez85@hotmail.com