Here at my SO we carry the Glock 21 Gen 4 pistol. One of the Sgt's has developed an issue with his pistol. In Dec 2017 he came to me with a magazine that would drop the top round out when removing the mag from the weapon. The following round would pop into place. I replaced his magazine, marked the old one and put it in the training bin for future testing. Today, March 2018, he brought his pistol in with the same issue on the new magazine. This weapon is three or four years old, is decently maintained, and has not been abused. The slide stop lever appears to be okay, the magazine does not have any issues that I can find. What am I missing? Is it possible that he is doing something that is causing the issue? And has anyone else had this issue?