My Greenstar 532i doesn't reliably fire up and heat the rads?

I suspect there is a problem but when we got the gas folk out before they just filled it with a bit more water (I think) and didn't indicate there was anything wrong . I have put the therostat control to manual, the flame signal is on and the WR1 unit by the boiler went to green. The boiler flame light is off and the number showing is 63 if that helps. But still no firing up. It is due another service so I would like to be able to wax knowledgable and make sure any fault is properly remedied. I would welcome any suggestions.
(I took the WR1 and thermostat unit through the 'learn' process and no red light 7 second flickering occurred.)