Surprised they aren't cutting out the middleman and handing checks directly to the Kerry, Pelosi, Biden, and Romneys
It could due to a couple of things such as the mag spring condition, follower, and sometimes ammo. It could also be a shitty magazine in general (It does happen). For ammo example, 300blk will load into a 556 mag, BUT, you will need to slam that in there really hard to seat properly.
just my experiences ymmv