Combitek F23E?

On set times to start Central Heating from the clock
the pump starts but burner does not ignite.
If I turn the boiler off and on again the burner comes on.
Is the problem something major?
The Main PCB has been renewed
Now I am made to believe the fault may be Control PCB
This all started from a small drip on the reversing valve
After repairing the valve with an o ring ?
the boiler did not start This led to installing the main PCB