I just got an AR in trade and the rear takedown pin wont come out or go in without tapping with a hammer. It is an Aero lower and a yankee hill upper. I took it apart and put the upper on my M&P15 lower and put the M&P upper on the Aero lower and everything fit fine and smooth on both assemblies. When I tried to put my new one together, the same thing, have to tap the pin in or out. Looking at it, the holes seems to line up ok. Will it loosen up with time, or should I worry.