I had that problem years ago.
Below the furnace where the air enters the ducts there is a tin, wedged shaped deflector that sends the air to either side of the ducting so the air is more evenly distributed throughout the house.
In my case it had come loose and was blocking at least half of the air leaving the furnace.
I had a guy come out and fabricate a better one and then we had air to every part of the house.
Hard to describe, but think of a V turned upside down, splitting the air flow. Best I can think of.
1987 Craftsman Double Wide 42x28,w/attached 28x12 foot enclosed porch/ re-shingled 2 yrs ago. Original exterior vinyl w/no sheathing.