AC running but not cooling: what to check, in order

The two you can check yourself

The filter comes first. It is the most common cause, the cheapest to fix, and a severely loaded filter can restrict airflow enough that the evaporator coil ices over — at which point the system runs continuously and cools nothing.

Then walk outside and look at the condenser. Its whole job is to dump heat into the outdoor air, and it does that through fins that need clear airflow. Packed with cottonwood, grass clippings or leaves, it cannot reject heat, so the refrigerant never gives up its load and the indoor air never gets cold.

Where it becomes technical

Dirty coils, duct leakage into an attic or crawlspace, a failing compressor and low refrigerant all look identical from inside the house: the system runs and the house stays warm. Telling them apart requires measurement — pressures, temperature split across the coil, static pressure.

This is the point at which guessing gets expensive, because several of these have very different costs and only one of them is the actual problem.

The refrigerant point, because it matters

Refrigerant is not fuel. It circulates in a sealed loop and is not used up in normal operation. A system that is low on refrigerant has a leak, full stop.

So 'it just needs a top-up' is not a diagnosis. Adding refrigerant without finding the leak restores cooling for a while, vents refrigerant to the atmosphere, and guarantees the same call again. Ask for the leak to be found.