How old is that particular magazine? With my issued Glock 19, my agency has replaced our magazines 5 times now, we are on mag group number 6 now. Yes Glock decided that the other mags were not up to snuff and replaced them all. It could be the same with your mag. I am not aware if they do the same thing commercially as they did with our mags, they placed a small number on the follower to let us know which series of magazine each was. I also don't know if Glock mags in pistols other than the model 19 suffred the same problems, but I do know that an early production problem with the amgs (at least through the first couple or few replacements was that the mags were not dropping from the magazine well as they should have been). In the earliest mags this is because they were made completely of polymer without a steel lining.
A good idea might be to contact Glock about this problem, whether or not the mag is new or old, and let them figure it out.
All the best,
Glenn B
