I found this elsewhere, and I'm happy to say, that's exactly what was causing my issue:
Problem: After firing the last round, the slide doesn't lock back.
Likely Solution: Lower your grip a bit/be conscious of where your thumbs are. Sure enough, mine was riding atop the slide lock lever. Adjusted my grip, and it runs like a top every time.
Posted with the subject as it is, to make it easily searchable.