Dampers.

A damper is a plate inside a duct or a register that pivots to restrict airflow. Three kinds turn up in houses.

Register dampers are the small lever on a supply vent. Anybody can reach them, and that is the problem: somebody closes one in winter and nobody opens it in May.

Balancing dampers sit in the branch ducts, usually with a small handle and a locking wingnut. They are set once when the system is balanced and are not meant to be adjusted casually.

Zone dampers are motorised, controlled by a zoning panel, and open or close whole sections of the house on demand from a thermostat.

When a damper is the fault

A single room that went from fine to poor, with no change anywhere else, is a damper or a duct problem far more often than it is an air conditioner problem. The one room guide walks through telling them apart.

Zone damper motors fail, usually closed, which starves an entire floor or wing while the rest of the house reads normal. On a two storey house that presents exactly like upstairs not cooling, and the fix is a motor rather than anything to do with the system.

Balancing dampers drift or get knocked by anybody working in the attic.

The thing not to do

Closing dampers or registers in unused rooms to push more air elsewhere is intuitive and wrong.

It raises static pressure across the whole system, which reduces total airflow, which starves the evaporator coil. Starved airflow is one of the two causes of a frozen coil, and a frozen coil is how compressors get damaged.

If a room needs more air, the fix is on the supply side to that room, not by strangling the others.

Finding one

A balancing damper handle in an attic is visible if you know what you are looking for. A zone damper motor can be tested by watching whether it moves on a call. Both are part of what gets looked at when a single area of a house will not cool.