The answer to 'why is my furnace not cooling' is that the most common causes are a refrigerant leak, a problem with the compressor, or issues with the air conditioning unit itself. Symptoms may include warm air coming from the vents, the compressor not running, or the unit freezing up.
If your furnace is not cooling properly, it's important to first check the air filter and clean or replace it if needed. You should also make sure the thermostat is set to 'cool' and the temperature is set lower than the current room temperature. If those simple fixes don't work, you may have a more serious issue that requires a professional HVAC technician to diagnose and repair.