What size is the current AC?
Have you pulled any of the covers off and looked in the plenum? Some times the plenum gets disconnected. They attach them with tape, and some times it's not a very good job.
Have you looked in the end ones to see if they even locked the end of the plenum. Maybe it's open and just blowing up into the attic?
Have you taken the cover off the AC and looked to see if it's sealed between the cold air exhaust and the return air?
That has been found at fault also.