Maybe they forgot to install the duct. Stranger things have happened. Pop one of the circular vent off and make sure there is duct. Then drop the lower cover and make sure it is properly taped. The front registers are fed off one side of the AC, the rear from the other side. One side should mirror the other. If not, break out the duct tape. This would also be a great time to tape up the ducts around the ceiling registers where they cut big circular holes for the registers. There should be no leaks into the roof area.
