What he said.
It would make sense that the butter aroma could be covering up your hop presence. Make sure to do a full on soak with PBW on your keg (that you have pulled completely apart, and soaked all of its parts), hoses, racking canes, carboy/bucket, basically anything that touches your beer on the cold side. Also try to do all racking into other vessels with a plastic wrap or tin foil that has been sanitized to cover the keg openings and carboy openings. The soak or spray all those things with starsan or iodophor. Also, since it is easy to oxidize those hop flavors and tastes, make sure to purge your keg, racking lines, and stuff like that with co2. Shat talked about doing this on one of the first Mini Topics.
Hey, what ever happened to the Mini Topic anyway?