either the magazine follower is sitting a touch crooked in the magazine, or your off hand w/ a proper combat grip is resting close enough to the slide lock bar that it is keeping it from springing up fast enough to lock the slide back. #2 is what my problem is when that happens to me. My offhand thumb indexes to the takedown lever and is pressed in against the slide frame which puts drag on the slide lock bar resulting in the gun going into battery w/ no round in the chamber.