My brand new HK 13 round mag for a USPc/P2000 (9mm) will not lock into place when inserted. It pops out unless I really smack it into place. I have this problem with this magazine in a HK P2000 and with the USPc so its an issue with the magazine not a particular gun. What might this be? As I mentioned it's a new magazine. It has worked fine till just a couple of days ago.
It may be because it's new?? My Taurus PT145 Millenium Pro does similar. With a full mag, it sometimes won't lock in, unless I push it in very firmly. Try loading one less round in it and see if that corrects the problem. Another thing to try is with the gun in slide-lock. If it inserts easily then and stays in, then the mag springs just need breaking in.