Come on folks, I just don't see a pistol in that picture. Let me look again......now I see it! All kidding aside, if the one extra round in the magazine is the difference in the magazine not seating, my only thought is that the mag is bulging just enough to prevent it from seating. Plus when inserted, the slide will put downward pressure on that top round. I know I have had magazines in the past that were a dog to get that last round in. And when I did, the mag was difficult as heck to seat to the point where I downloaded the magazine by one round.
Hope you get it sorted out. I've got a S&W Shield Plus 13-round magazine I can only get 12 rounds in. Not going to push the issue, but hopefully through use I can get that last round to go in that mag.