[jimcol]

Our is an older model and we experienced a similar issue when it was new. Living room floor vent closest to furnace not much air other 5 vents were fine. Furnace was heating the unit just fine. Talked to service manager about it. His explanation, "There is a baffle, (actually just a bent piece of sheet metal in the main duct run), just ahead of vent 1. It pushes the majority of the air past the first vent to the remaining vents in that run. He said without it too much air goes to first register not leaving enough to adequately heat the rest of the main floor." Didn't make a lot of sense to me but with a flash light and a mirror I was able to see the baffle. I do seem to remember the three upstairs ducts were connected via two flexible tubes off the plenum. One direct to the bathroom vent and one into the pan duct supplying the upstairs bedroom. I did have to reattach the one flex duct to the bathroom register years into ownership.
It's heated fine for 15 years so I guess it working as designed. (and we have been in some cold climates over the years) If it is warming the unit OK, I wouldn't get hung up on the air flow. These things are not like a house heating system and are not very efficient but they do the job they are designed to do. Let us know your findings.