Somebody once posted they had the opposite problem, their slide wouldn't lock back after the last shot. Royce told him to shoot it left handed and see if that fixed it, which it did. It seems his right thumb was resting on the slide release just enough to keep it from engaging. I would think that if you are hitting the slide release from underneath and pushing it up into lock position that you could test it out by also shooting it left handed. If the problem goes away you'll know it's your grip and can make adjustments as needed.
Good luck,
Bronson
"Anybody worth shooting is worth shooting five to seven times"
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