[HayZee518]

long runs the furthest from the heat source may be too small and if flexible runs, the fiberglass inside the duct may be absorbing all the air flow. have you thought of replacing the long runs with galvanized trunk lines? this way with a galvanized run you can install a booster squirrel cage fan in the longest runs. cold or hot air returns don't follow no cut and dry rule, they just nail up a sheet of sheet metal on one joist space then duct it to the furnace or a/c unit.