The Natural Philosopher submitted this idea :
In general especially if there is hard water and not enough inhibitor these
valves get sticky. You can remove the head and access the valve spindle and
see if it turns smoothly and with little effort, without having to dismember
the plumbing. Sometimes working it a few times manually clears a bit of
blockage. Other times its is simply all gunged up and its a whole new valve.

The spring return type actuators jam, much more often than the actual
valve. Easy to check- the D spindle of the valve should be able to be
turned, almost with your finger tips if it is free. If not, work it
back and forth a few times to free it.