I'd bet that you're not getting the nozzle to seal in the bucking, or the piston isn't sealing in the cylinder.
To test the cylinder, remove the outer barrel, cock the gun, and shoot while you plug the nozzle with your thumb.
If the piston stops, then it's likely between the nozzle and bucking. If that's the case you'll either want to put a narrow band of scotch tape on the inside of your hop chamber, above the feed lips of your bucking. This will squish the bucking inwards a little bit, providing a better seal on the nozzle.
To test the cylinder, remove the outer barrel, cock the gun, and shoot while you plug the nozzle with your thumb.
If the piston stops, then it's likely between the nozzle and bucking. If that's the case you'll either want to put a narrow band of scotch tape on the inside of your hop chamber, above the feed lips of your bucking. This will squish the bucking inwards a little bit, providing a better seal on the nozzle.