chk that tire pressures are to spec and equal side to side. then level deck to ground from bottom of blades per manual. operate at full rpm and slow ground speed.

see if above helps.

I would be concerned if something was changing the air flow below the deck. I don't recall if there much of a skirt at the front of the deck housing. The deck should be clean and blades not horribly worn. There is a baffle too which divides the front of the deck to kind of segregate the 2 blades to even the lift. There is also an anti blow out kit for these that are meant to improve bagger performance and keep that from happening as well. The common kit is part # 114-1620 > You could look it up with the 5 digit model number from the frame under the seat or possibly the bagger.