FWIW ...
I had a right-handed fellow instructor bring me a Sig Sauer pistol a while ago, reportedly because the slide consistently failed to lock back when the magazine ran empty during shooting.
I took it out to the firing line and shot it. It didn't do it for me, even though I'm right-handed. It did it when he shot it.
Now, admittedly, being an armorer and shooter of a number of different pistols, some of which have the slide stop levers located far enough to the rear of the frame to place the engagement surface of the lever adjacent to the ball or tip of my thumb, I've adjusted my grip to keep my thumbs away from ALL slide stop levers.
I finally had the fellow shoot the pistol left-handed. It functioned normally, with the slide consistently locking back when it ran empty. It took a while for him to actually understand ... or believe, I suppose ... that it functioning normally when he shot it left-handed (and his thumbs were on the right side of the weapon), but repeatedly failed to lock back when he shot it right-handed, wasn't just a coincidence ...
Naturally, there could be mechanical issues involved in any situation where functioning problems arise ... but shooter-induced/related functioning issues occur much more frequently than many users/owners would probably care to believe.
The good news is that sometimes 'fixing' the owner/user means there isn't a mechanical problem with the firearm ... and the bad news is that getting the 'fix' to stay 'fixed' requires the owner/user to make it so.
