The answer to 'why is my heat pump not heating' is that there could be several common issues causing the problem, such as a refrigerant leak, a malfunctioning compressor, or a problem with the reversing valve. The heat pump may not be able to extract enough heat from the outside air, especially in very cold weather, or it may have an electrical or mechanical failure that is preventing it from producing adequate heating.
Some common symptoms you may notice include the heat pump running constantly but not heating your home effectively, the air coming from the vents feeling lukewarm or cool instead of hot, and your energy bills being higher than expected. If you notice these issues, it's best to have a professional HVAC technician inspect your system to diagnose and repair the problem.