There are several videos on YouTube addressing the AC loss by checking the unit from inside. They all seem to tape and smooth corners and crannies that are not installed correctly.
Then I would try the old fish tape and flashlight search and seizure of junk in the ducts.
At the ends of the ducts, some folks place a block of sorts to stop all movement of air past the furthest opening. Some say the ducks just open into the ceiling cavity. I did that and it seemed to improve air flow into the bedroom area in a Keystone product.
Whoooo knows? You might find a treasure up there.