The problem is the magazine follower. On the front of the follower is a shelf that engages the slide lock. In the instances where the slide lock won't engage the shelf goes past the tab on the slide lock. In other words the shelf isn't wide enough to engage the slide lock. This can be checked by removing the slide and inserting the magazines that are problematic and observing the contact between the shelf on the follower and the slide lock tab. This appears to be a design flaw as this can happen with a brand new magazine. I haven't seen a magazine for this gun with a metal follower which could be better than the plastic ones that I have. Also the slide lock tab could be made longer. Hope this is helpful.