I don't know if this might be a possible cause or not, but I found with a couple of my mags, a 20 and a 30, that the last round would not insert. After several attempts of trying to load the mags I discovered that the mag spring was bunching up at an odd angle as it compressed and it wouldn't let the last round be loaded. I found that tamping the mag on a towel about every fifth round kept the spring in alignment and the rounds loaded fine. I know that if I had really tried to force that last round into the mag I could have possibly broken something.
