I have a PSA ar15 with the metal mag that came with it and a magpul mag that I bought extra. When I insert the metal mag and I pull the charging handle, it will chamber a bullet. But with the plastic magpul, when I insert the mag and pull the charging handle it does nothing. The mag want even catch in the mag well. I have to lock the bolt catch for the mag to catch in the mag well then release the bolt catch to chamber a round. Why is this? Is it because one is metal and the other plastic or what. I know this may be a newbie question, but thats what I am. New to the ar platform. Thanks for any help.