Is your dash coolant temperature gauge reading as normal? Thermostat unlikely as coolant flows around heater before thermostat opens. First ensure you do not have an airlock in the system. Warm engine up, set heater to max temp and pump top hose with your (gloved) hand. If no air lock is present it is likely to be a problem with your coolant valve, which is found behind the scuttle cover panel in the engine compartment. This unit controls quantity of hot water admitted to heater matrix. Could be an electrical fault or the valve itself. Not too tricky to do.