Same issue I had about a year ago, mine was the upside down U shaped plastic bb stopper inside the hopup. The sharp ridges on the cylinder head rubbed the heck out of the plastic bb stopper and it stopped, stopping bbs,lol..
The bb stopper is a bit before the bucking so the bucking wouldnt be the reason.. infact the only thing if the bucking wore out it would just take in the bbs and they would roll out of the front of the barrel.. I searched and searched all over when my bb stopper went bad and I couldnt find one on one single site.. Anyplace! I had to Ranger rig one from an old rifle I had years ago and it wasnt a perfect fit so I had to razor knife it till it worked.