Do you have access to another brand of BBs to try? It sounds like the mag does not want to reliably push the BBs upward, which is either a weak spring (unlikely on a new mag) or the BBs are binding in the channel (could be against each other).
Some people recommend a spritz of silicone spray into the BB channel to make sure the BBs feed smooth, but then you need to blow a mag or two of BBs out to clear the oil. Makes a mess in my opinion. I just use highly polished BBs (BBBastard, Green Devil).