The heaters are fuel fired units, so a lack of fuel could be the reason why the heater is not operating as required. Within the heater's fuel line system is a "Fuel metering pump" that contains an internal fuel filter. Refer to the technical for location of the filter.
This small pot shaped filter may need cleaning or replacing. A blocked or restricted filter will not allow the heater to run efficiently and maximise the potential heat energy available. Should the boat or vehicle be subjected to rough weather or travel over rough ground, sediment in the bottom of the tank may have become disturbed and is now blocking the metering pump filter. So please check this out.
This problem usually revolves around lack of fuel being delivered into the combustion chamber. It always worth checking out the comments made in Q3 for air and water heaters. Your heater may be overheating.
It could also be the thermostat or modulator is set too low, ensure that the thermostat or modulator control is adjusted up to a higher setting.
Check that the fuel metering pump is working, this is confirmed if the fuel metering pump is "ticking" also check that fuel is being delivered to the heater fuel inlet connection, and check all fuel line joints for security and that they are not either leaking fuel or allowing air into the fuel system.
Check the fuel level gauge, have you run to low on fuel in the main fuel tank?
Check that any heater fuel shut off valve is in the open position.
Inspect the exhaust pipe or combustion air tube for any restrictions