The answer to 'heat pump not cooling' is that the most common causes are a refrigerant leak, a problem with the compressor, or issues with the air handler or ductwork. Symptoms may include the heat pump running but not cooling the home, warm air blowing from the vents, or the compressor not turning on.
If you suspect your heat pump is not cooling properly, first check the air filter and clean or replace it if needed. You should also make sure the outdoor unit is free of debris and that the refrigerant lines are not damaged. If those simple fixes don't resolve the issue, it's best to call in an HVAC professional to diagnose and repair the problem, as they have the tools and expertise to properly service a heat pump system.