The answer to 'heat pump not heating' is that there are several common causes, such as a malfunctioning reversing valve, refrigerant leaks, or issues with the compressor or fan. Symptoms may include the heat pump running but not producing warm air, or the system cycling on and off frequently.
In many cases, the solution may involve repairing or replacing faulty components. However, it's important to diagnose the root cause properly to avoid further damage. For most homeowners, it's best to call a professional HVAC technician to inspect the system and make the necessary repairs.