It sounds like you've done the obvious external stuff other than perhaps checking the crown.Your problem could be internal - like a defective piston seal. Unless you're up to replacing that it may be time to return that one.The problem could be pivot related as well - as in side play.The gun SHOULD be capable of half inch five shot groups at twenty yards, though perhaps not consistently.
if group is wide horizontal left to right.. its likely play in the pivot screw or washers part #20http://www.crosman.com/vantage-np-with-4x32-mm-scope-177in dloads get 30021 Exploded View of Parts Diagram (EVP)i replace it with thrust bearings and now my group is just up and down for pellet speed inconsistencies.