My Sig P226 won't lock the slide back for either my daughter or my wife - I've gone over proper grip and stance all day long - I pick up the gun (same ammo/mags, etc...) and it works fine, they pick it up and no slide-lock back on last round.
Limp-wristing will do crazy things to a gun.
Sigs are a whole other issue. They are probably pushing down the slide lock with their thumbs while you are not. None of my da/sa sigs lock back for me because I ride the slide lock with my thumbs. My sao sigs work fine though.