I purchased a used G21 that only came with one magazine. I purchase two more OEM magazines from my local gun store. I also purchased a G20 conversion barrel and another 2 OEM G20 15 round magazines. When I went to the range I discover that none of the 4 new magazines will lock into the pistol. They all slide right back out on their own after being inserted. After a close inspection. I noticed that the one magazine that came with the pistol had a metal plate on the front of the magazine that the 4 new ones do not have. The interesting part is that all 4 new ones will lock into my G30 without issue. Can someone please tell me what I am doing wrong?