The answer to 'HVAC system not cooling' is that there could be several common causes, such as a refrigerant leak, a problem with the compressor, or issues with the air circulation. Symptoms may include the system running but not producing cold air, the air coming out of the vents feeling warm, or high humidity levels in the home.
Some potential solutions include checking the air filters and replacing them if they are dirty, clearing any debris or obstructions around the outdoor unit, and ensuring the thermostat is set correctly. However, more complex issues like a refrigerant leak or compressor failure may require the expertise of a professional HVAC technician.