The short version is if you have a part of your house, let's say it's just one room that is warmer than the rest. Always right. So that part of the house seems to get whether it's because heat rises or because of some other reason. Maybe it's near something else, creating heat or whatever, but that room is hotter than the rest of the house or warmer than the rest of the house the majority of the year.
Whether it's in AC mode or heating mode. in that case, it is going to be a zoning issue.
Manual dampers at the plenum will help too.