What has been missing in all this discussion so far is any person in your home performing a Total External Static Pressure (TESP) test. This test can find more airflow problems in a shorter amount of time than any "guess and check" method.
Originally Posted by backpacker
Visual inspection of the supply ducts can advise a tech of potential problems, such as leakage, kinked and/or sagging runs, duct sizes, length of runs, etc. But you need someone competent who can do both a visual assessment AND understand results of a TESP test. AND then know how to advise you accordingly.
What type of duct material does the problematic system have?
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.