It sounds like the mag is sitting too high. The top of the mag well in the upper has a restriction that prevents over insertion, this is what was making the mag hard to insert. Either the catch is too high, or the restriction is too low. Could you attach some photographs of the receivers?
The mag sitting too high will cause the lips to interfere with the carrier, preventing it from moving into battery.
It sounds like a scaling issue, pictures might help find where.
