The answer to 'why is my evaporator coil not heating' is that the evaporator coil is likely not getting enough refrigerant flow. This can be caused by a refrigerant leak, a problem with the compressor, or a blockage in the refrigerant line. Without sufficient refrigerant, the evaporator coil cannot absorb heat from the air and warm up as it should.
Some common symptoms of an evaporator coil not heating properly include the air from your vents feeling cool but not warm, the coil itself feeling cold to the touch, and possibly ice buildup on the coil. If you notice these issues, it's best to call an HVAC professional to diagnose and repair the problem. Attempting to fix the issue yourself could be dangerous and may cause further damage to your HVAC system.