This is fairly common with Glocks (and certain other guns), and it's almost ALWAYS shooter-caused. If the user has their thumb too close to the slide lock/release lever, their thumb will push up on it during the recoil cycle, causing the slide to lock back prematurely (and you know, everyone hates premature slide-lockulation).
Originally Posted by SeanB1986
You should also know that, once presented with the diagnosis of the problem, the shooter will usually swear on a stack of bibles that their thumb was NOWHERE NEAR the lever, and that this COULD NOT POSSIBLY be the problem. When it happens to some folks, it happens several times per magazine. Once you convince them to move their thumb completely away from the lever, it stops.
There is at least one gun problem, caused by incorrect assembly of a Glock, that will also induce this malfunction. However, if that is the case, it will occur on EVERY SHOT until the weapon is reassembled properly. Intermittent problem = shooter-caused.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)