First, the obvious question. Are you positive your thumb (rt. handed shooter) is not ever so slightly touching the slide lock lever while firing. Recoil does strange things to one's grip and any contact with that lever could prevent the slide from locking back after the last round is fired. Easy test. Fire a few rounds left handed, or grip the gun in such a manner it would be impossible for you to have contact with the slide lock during firing.
I shoot a fair amount of 1911's and my thumb rides the safety. No problem. Transitioning to, especially my Sigs, requires a slight grip change. I stopped worrying about the slide locking back on the last shot years ago. I am too old to retrain.
"Never pet a burning dog"