Start where the failures actually live. Airflow first: a filter past its life, a blower struggling, or returns blocked by furniture starves the coil, drops cooling, and eventually freezes it. A surprising share of no cooling calls end at this step, which is why any diagnosis that skips it and opens with a compressor is out of order.
In San Marcos, read a not cooling call against the inland thermometer. A system that holds the house on a mild coastal style day and loses it at 3 p.m. in an inland heat event is telling you about capacity, charge, or airflow in that order, and the technician's readings should say which before any part is quoted. The local urgency is real: when heat settles in here, it stays for weeks, and a marginal system does not catch up at night the way it would at the coast.
Electrical second: the capacitor and contactor are the small parts that start big motors, and they fail on the hottest days because that is when they work hardest and longest. Gauges third: refrigerant charge is measured, and low charge means a leak to find, because refrigerant circulates in a sealed loop and is never used up.
Ducts last and often: cooled air dumped into an attic through a separated run cools the attic faithfully while the house complains. If one end of the house never cools while the other overcools, suspect the air's road, not the machine making it.
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