The slide on my Glock won't lock back?

My slide won't lock to the rear without pulling it all the way back and pushing up on the slide release to manually do it. I just loaded it for the first time and fed all the the rounds through to check and be sure these HP's feed ok, which of course they did. But then the slide stopped locking back. It worked last night just fine, but since I did that it doesn't. Anyone got any ideas?