IF you hold the mag up and it feeds fine -It's not the bleepin tappet plate. ON a hop up the tube part that sticks down does two jobs 1. is the path for the bbs to arrive in front of the nozzle and 2. pushes the release catch/wedge back on the top of the mag which allows bbs to exit the mag. In this case this is only happening when the OP physically lifts up the mag gaining a tiny but crucial extra bit of mag height. Therefore he needs a mag release that holds the mag up higher or some way of holding the hop up unit down a bit or to build up the length of the lower part of the hop up unit slightly. Use your heads people ... If it was me i would put some jb weld on the top of the "wings" (ridges moulded onto the side of the hop up unit). This will cause it to sit slightly lower

A strip of tape or two (like metalic duct tape) on the top of the notch in the mag may be enough ad well. I had a jg m4 that needed tape on all my mag brand vn mags.

Well it does depend on in what position the tappet plate is pinched in. If it's pinched in the forward position then it wouldn't feed at all. But if it's pinched in the rear, then this could explain the issue very well. But it's hard to determine while firing the shot, due to not being able to judge the approximate range difference