a sliding door encasement is pretty air tight. When air leaks through it can pretty well be caused by a build up of debris in the track, the felt striping has come off of the doors leading edge, or abuse.
Now, you might think that the air is coming in from the actual door but it could be around the actual door behind the trim molding. The install job might not have included stuffing insulation around the outer door jamb, upon inspection by pulling off one trim molding you might find nothing there. If so, remove all the trim and cram insulation all around the jamb this will be an overnight improvement.