If it's loose in the hopup chamber IE it's sliding too easily there you can add just a little wrap of teflon tape around the bucking being careful not to cover the area where the nub/arm pushes down on the hopup bucking and also missing the area where the C-clip attaches the whole thing to the barrel. That will make the fit a bit more snug which should also help with air seal in that area.
Before putting it all back in the gun though take the assembled barrel/hopup look through the hopup end with the hopup fully engaged and make sure the nub on the bucking is perfectly centered in the barrel. Aside from a bad bucking, or torn bucking that should solve any problems with it going one way or the other.
Another thing to watch for is a problem with the outer barrel being bent which can cause all sorts of these issues, or some defect in the inner barrel, or some BB residue in the inner barrel. None of which is nearly as common as the bucking being misaligned so I would certainly start there.
Luke
