How to fix huge temperature difference in central A/C

[aaa111]

I have a central A/C via heat pump connected to a blower furnace in the basement of a 1-storey building. I've noticed that year to year, the main floor is getting less and less cool and now there is a 10-degree celsius difference between the 2 floors. Basement is 19C and upstairs is 28C. Is it the duct work that's the problem (although heating in winter is fine - I know heat rises)? The heat pump is about 15 years old now. Would getting the ducts cleaned be of use, or is it time to replace the heat pump?

[astroboi]

Are all of the cold air returns (intakes) clear? Do you have any air intakes in the basement? Have you changed the air filter? What temperature is your thermostat set at? Have you closed the basement registers?

[Pete_Coach]

Close the dampers in the basement ductwork. This directs the cool air to the remaining vents.
Also, cool air is heavier than warm and the fan has to work harder to push the cold air so, if there are any obstructions in the air path (dirt, filters etc) the amount of air flowing will be be reduced.