The answer to 'why is my ductless system not heating' is that there are several common causes, including a refrigerant leak, a problem with the compressor, or an issue with the thermostat or control board. Symptoms may include the system not producing warm air, the compressor not running, or the system shutting off unexpectedly.
To troubleshoot, first check the thermostat settings and ensure the system is in heating mode. If the thermostat is working properly, the issue may be with the refrigerant level or a problem with the compressor. These are complex issues that typically require a professional HVAC technician to diagnose and repair. Attempting to service the refrigerant system or compressor yourself can be dangerous and may void the warranty, so it's best to call a licensed HVAC contractor.