FYI, jsut because you can see the lever move on outside of the valve does not mean it is working. Majority of time when these fail, the actuator lever moves but has come loose inside and valve is stuck. Try running some water through it with voltage connected to see if it is acutally opening and closing.
Pressure to the system comes from the engine oil pump, if you don't have pressure or flow to feed the heater the engine would be showing low oil pressure. If thermostat was stuck, little or no heat to the heater but then engine would be overheating after warm up. If engine is low on oil could affect heater preformance.