Airsoft gun mis-feeding when there's pressure behind the BBs. What could be causing this?

My JG Ak74 just got a few upgrade parts, including a new air nozzle which is .9mm shorter than the stock one, so I have been having many problems getting it to compress correctly. I shaved off .3mm of the tappet plate to allow the air nozzle to go forward slightly more, bettering the compression and hopefully solving my problem, but now the problem is that the gun will only drop BBs out of the barrel or lob them a few feet when a wound high-capacity mag or mid-capacity mag is inside, but if I turn the gun upside down and have either a barely wound high-capacity mag or manually feed BBs into the hop up chamber so they just fall gently into place, the gun shoots fine.

What would be causing the mis-feeds only when BBs are being forced into the hop up chamber? It isn't double shooting, just jamming up or letting BBs fall out the barrel.

I have already taken the gearbox apart 12+ times in the past 48 hours to troubleshoot, so ANY information to save me from more of that would be greatly appreciated.