Most likely you have an air pocket in the cooling system.
Let the engine cool down, open the bleed screws and fill the system. close the screws, turn on the heater and start the engine.Watch the temp guage and see if you are getting hot air from the heater. If the heater is cold and the temp is hot, shut off the engine and allow it to cool down again..repeat until you get heat from the heater.
or you could buy one of these
Cooling system evactuator