Rooms are getting hot at different rates

I live in a tri-level house (not a three level but a tri-level - main floor, and a split basement + top level.

When we turn on the gas heaters (air cooling system), one of the rooms in the upper level is easily 7-8 F above the rest of the rooms. Because of needing to keep that room liveable, the rest of the rooms end up being colder and uncomfortable. I do have temperature sensors in a couple of rooms that I move around to measure it. I am unsure why there is so much variation. The house is over 50 years old, and the HVAC system was redone sometime in the past 15 years.
What should I be looking out for to understand why the one room is so hot? I was told by a neighbor to get the duct seal redone, but I am unable to understand how only one room is way too hot than the rest of the rooms.