[L473ncy]

Possibly a mag catch issue? This is just a wild guess but maybe the mag catch is slightly off spec and doesn't let the mag sit right all the way up therefore doesn't push the BB's up into the hop up unit.

Other thing I'm thinking is that you could shim the mags in a way that it sits "higher" so that it will push the BB's up. When you hold it upside down and it fees, it could mean that the springs in your mags aren't pushing up hard enough and only feeds because gravity is helping it.