Heat rises, so a little difference between floors is normal. A ten-degree difference in July is not. When we get that call in McDonough, Johns Creek, or East Cobb, the house is almost always a two-story built between the late 1980s and the mid-2000s, and the attic tells the story within a minute of opening the hatch.
The first thing we find is thin insulation. Attics that started at ten inches have settled to five or six, and around the hatch and the HVAC platform they've been walked flat. The second thing is air leakage: gaps at the top plates, around can lights and bath fans, and where the duct boots come through the ceiling. Every one of those pulls 130-degree attic air toward the rooms below.
The third thing, in a lot of these homes, is the ductwork itself running through that hot attic with insulation that's torn or missing. The air handler is working, but it's fighting the attic the whole way.
The fix is in that order: seal the leaks, then blow the floor back up to a current R-value, and address the ducts if they're the problem. Most of these jobs take one day, and the upstairs difference is usually gone that evening.