like Pepa said check that tappet plate. If that is cracked in half (happens
) then it may be stuck in the far rear position meaning bbs are being forced into the chamber by the spring of the mag, and then randomly shot out with the airnozzle-tappet plate all the way back. This means all of the air forced out of the gearbox thats supposed to drive the bb gets mostly routed out the magwell.
You will of course have to take the gearbox apart to replace the tappet plate.