I've had this issue periodically. It happened once at the ARPC IDPA match on Saturday. Here's what normally happens. My Smith & Wesson M&P Pro 9mm goes to slide lock. I reload by inserting a new magazine. I shove it in hard enough to cause the slide to go forward when the magazine is inserted. Usually a fresh round is stripped from the new magazine and the gun goes bang when I pull the trigger. With a good reload all of that takes less that 2 seconds between shots. When things go well.
But maybe once every 20 reloads, I do all of this and no round is chambered after the reload. I've thought that this happened because I slammed the magazine in too fast and hard. But I don't think that's the source of the problem. Thanks to Dan's suggestion at the match this Saturday I checked the M&P Forum to see what others have said about this.
The M&P has an ambidextrous slide stop release. And it's easy to depress this when you are performing a lightning fast slide-lock reload. That sounds right to me. Now I have to pay attention during my next practice session and see if this might be my problem. I suspect that it is. But I want to confirm it in live fire practice.