The answer to 'why is my heat pump not heating' is that there are several common causes, including a refrigerant leak, a malfunctioning reversing valve, or an issue with the compressor. Symptoms can include the heat pump blowing cold air, not producing enough heat, or making strange noises.
Some potential solutions include checking the refrigerant levels, inspecting the reversing valve, and ensuring the compressor is functioning properly. However, many heat pump issues require professional diagnosis and repair, so it's generally best to call an HVAC technician if you're unable to identify and fix the problem yourself.