The answer to 'why is my AC not cooling' is that there are several common causes, including a refrigerant leak, a problem with the compressor, a clogged air filter, or an issue with the thermostat. Symptoms may include warm air blowing from the vents, the AC running constantly, or the system not turning on at all.
If your AC is not cooling, the first thing to check is the air filter. A clogged filter can restrict airflow and prevent proper cooling. You can try replacing the filter yourself. If that doesn't fix the issue, you may have a more serious problem that requires a professional HVAC technician to diagnose and repair, such as a refrigerant leak or compressor failure.