The cause is most likely uninsulated and leaking duct work in the attic. If you have flexible ducts then they should be at least R7 insulation. Any rigid ducts can be covered with insulation batts.

Buy a digital probe thermometer & a humidity gauge for indoors.

Then take the actual temps along with reporting the indoor humidity level.

be careful in covering ducts with insulation.
in climates like mine (hot humid) this will cause
condensation at the vapor barrier of the duct wrap
(or outer liner if flex duct).
this is a climate specific detail.

I'd haul myself into attic & check it out for loose
ducts, restricted ducts & DAMPERS at supply
plenum.

best of luck.