The heating/cooling systems I have seen usually have dampers/flaps/valves or whatever you call them in the ducts where you can adjust the air folw to different rooms to help ballance the air flow.
Yes, good point, you can reduce or close the flow through some to get more air to areas that need it more. But you can only do that to some reasonable percentage before it's affecting overall performance negatively. Which gets back to what I said, I'd measure the temp in various areas. If they are all about the same, shifting it around isn't going to help, but if there is a diff between some areas, then you could do the adjusting. My guess from what we've heard is that it's either undersized, not running correctly, eg charge is wrong, or the duct work is inadequate. No one has mentioned filters, that should be checked to make sure they are clean and not some overly restrictive one. I suggested he see if the air going into the unit and coming out is ~18F difference. That's in the typical range. He could sample the air at the return intakes and some of the vents.
Good point on the filter. Friend had an upstairs room that was not getting much air flow. He had been having his heat pump serviced as it had an electronic filter. After 4 or 5 years he decided to do the servicing himself. Found out that someone had left in the regular type of filter that is usually changed once every month or so. It had almost all the air flow stopped.
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