Have the flow rate from your registers measured. Typically, there are big leaks in the ducting which sends very little air to the upper floors.
Nealtw is exactly right about the venting too. If the attic is unvented, the hot air will just sit there and heat your upper floors.
Lastly, you may have insulation deficiencies and poor windows. All of these contribute to a better HVAC systems.
Do all these steps then come back . . . we can talk then!