What is the "greasy" spot on a deer? Never heard that one. If you didn't find the deer or the arrow then you really don't know for sure where you hit him so I would assume from all indications that if you think you hit him far back then you probably did hit him in the arse. Regardless of carbon or aluminum, you should get a clean pass through unless you hit some solid bone and there is plenty of that in the arse area. If the buck was quartering towards you then any shot "below the backbone and left (forward) of the butt" will have an angle such that the arrow will carry back toward the arse. Nevertheless, I wouldn't suspect the reason you didn't get any pass-thru's would have anything to do with whether the arrow was carbon or aluminum.