The answer to 'why is my HVAC system not cooling' is that there are several common causes, including a refrigerant leak, a malfunctioning compressor, a clogged air filter, or issues with the thermostat or electrical components. Symptoms may include warm air blowing from the vents, the system running constantly but not cooling, or the unit freezing up.
If your HVAC system is not cooling properly, the first step is to check the air filter and clean or replace it if needed. You can also try adjusting the thermostat to a lower temperature. However, if the issue persists, it's best to call in a professional HVAC technician to diagnose and repair the problem. Attempting complex repairs yourself could be dangerous and may even void the warranty on your system.