The five things we actually find
1. Undersized supply ducts upstairs. The original builder used the same trunk-and-branch design for both floors, and the upstairs branches can't deliver the airflow the upstairs needs in July. We measure delivered airflow at each register with the balometer and compare it to what the room calls for from the load calc. The mismatch is usually obvious.
2. Not enough return air upstairs. Conditioned air gets pushed up but has nowhere to come back from, so the system goes pressure-positive upstairs and the air just sits there. A single upstairs return can fix this — but you need to size it for the airflow, not just cut a hole.
3. High static pressure on the system overall. Equipment rated for 0.50″ wc that's actually running at 0.95″ is moving a fraction of the air it was designed to. The upstairs feels it first because it was already at the end of the line. The manometer reading tells us this in a minute.
4. One system, two floors. A single-zone system in a two-story home was always going to fight gravity. Sometimes a bypass damper or a zoning retrofit is the answer; sometimes it's a small dedicated upstairs system. Sometimes — often — fixing the duct problems above gets you 80% there without adding equipment.
5. Attic heat infiltration. Hot attic air leaks into upstairs ducts through bad joints and uninsulated runs. Infrared imaging and a duct-leakage test confirm it. The fix is in the attic, not at the thermostat.
What won't fix it
A bigger outdoor unit. A smart thermostat. Closing downstairs vents (this makes static pressure worse, not better). Cranking the thermostat lower. Each of these is what we hear from homeowners who've been told to try them, and each one leaves the actual cause untouched.