I have a gen 3 G21sf (along with 5 other Glocks). I noticed when breaking down for cleaning that the rear of the stock guide rod and spring had slipped "down" and out of its notch. Usage is not what I would consider heavy -- approx 3500 rounds over 2 years. Its happening now after a few cycles, not just live fire... Have not had this happen on any other model. Has anyone had this happen, and how did you correct or fix?