Rooms that never settle usually have a duct problem, not an equipment problem
When one bedroom is always warmer and the unit seems to run forever, owners assume the system is too small. Often the air is leaking or collapsing on the way. Attic flex duct sags, kinks and tears over the years, and a leak in a very hot attic dumps cooled air into space nobody uses.
Whistling returns, vents that barely move air, and a return grille that tugs at a nearby door are the other signs. They suggest the ductwork is too small, crushed, or leaking at the plenum or the take-off collars. A brand new condenser on bad duct will still bill you for it every month.
