The answer to 'why is my air handler not cooling' is that there could be several common causes, such as a refrigerant leak, a problem with the compressor, or a blocked air filter. Symptoms may include warm air blowing from the vents, the air handler running constantly, or the system not reaching the set temperature.
To troubleshoot, first check the air filter and replace it if it's dirty. You can also try resetting the system by turning off the power for a few minutes. If the issue persists, it's best to call an HVAC professional to diagnose and repair the problem, as they have the necessary tools and expertise.