Looking at insulation is not an exhaustive test. However, it you are confident that the insulation is adequate then the forced air heating & cooling is not getting to the top floor. Probably because of inadequate duct work. Depending on the home, it can be very expensive to redo the ductwork. You may have adequate ductwork but the furnace blower may be undersized. There are a few variable here.
You can look into supplemental heating & cooling by installing a heat pump in the attic that services the top floor. There are probably several options to compare.