intermittent problem with central heating valve not opening?

We have a Drayton wireless for our central heating, and a Drayton ZA5679-2 2 Port motorised valve also for heating.
Our hot water side works fine.
Occasionaly when heating is called for, the sliding switch on the motorised central heating valve will not move to allow the valve to open to allow the boiler to fire up. nothing happens, no noise as if the valve is trying to open.
After about an hour this problem will go, I call for heat, the slider moves, valve will open and the boiler fires up. we have heat.
While this problem is happening we can manually move the slider on the valve actuator which opens easily the valve opens & boiler fires up. all will then work as normal.
This problem can occur if the boiler is already alight for hot water.
Where does the signal come from to tell the actuator/valve to open. I don't want to replace the actuator if the problem is elsewhere.
The wireless receiver looks like it is doing all the right things, green light will come on when heating called for.
If anyone has any suggestions as to where the problem might be. I would be grateful.
The gas valve on our boiler sometimes makes a chuntering noise just when the pilot light comes on just before firing up, a plummer has said this is not connected to the above problem.
I am in all day with heating on, 95% works as it should.
Thanks in advance.