The air filter housing cover has a rubber seal that sometimes sticks, if you are certain that all the bolts are removed, you might have to pry or put some muscle to break the seal and open up the cover to be able to replace the air filter. Lift firmly on the bolt side of the cover as the opposite side has tabs that slide into slots in the lower half of the air filter housing and you don’t want to snap these tabs off or break the retainer slots.
The cover lifts up slightly on the rear of the cover so it's not sealed, but on the front end toward the engine it slips into a rubber hose about 4" diameter and the is held on by a stainless steel adjustable hose clamp, that's the part that I don't seem to be able to get off. If there is a special process to get that off, I would appreciate some help. Chuck