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 airflow. Symptoms may include warm air coming from the vents, the system running constantly but not cooling, or the compressor not turning on at all. Solutions can range from simply changing the air filter to repairing a refrigerant leak or replacing the compressor, depending on the root cause.
If the heat pump is not cooling, it's generally best to call a professional HVAC technician to diagnose and repair the issue. While some basic troubleshooting like changing the air filter can be done DIY, attempting to work on the refrigerant system or compressor without proper training and equipment can be dangerous and may cause further damage to the unit.