On some models with the screw in valve, ( not sure if yours is the same ).
We have had some where the system tries to work but the pump just runs and nothing else happens as though they need bleeding, We Replaced the pump motor on the last one and stripped the original one to see what the fault was, The problem was caused by someone screwing the valve in tightly and when the valve has then been unscrewed, The Tip breaks off and stops it from building pressure up.