The answer to 'why is my air handler 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 or electrical system. Symptoms may include the air handler running but not blowing cold air, the house not cooling down, or the air conditioner freezing up.
If you suspect an issue with your air handler, the first step is to check the air filter and replace it if it's dirty. You can also try adjusting the thermostat to see if that resolves the problem. However, if the issue persists, it's best to call a professional HVAC technician to diagnose and repair the problem. Attempting to service the air handler yourself can be dangerous and may void the warranty, so it's generally not recommended for homeowners to do major repairs on their own.