Potterton Flamingo 40 - Pilot light stays on but does not flare up?

The pilot light stays on, but even when there is no hot water, the boiler does not flare up to start heating the water even at Maximum level. There is normally a click when you rotate the temperature valve but it has stopped occurring when you rotate it. I have tried to switch off the pilot light and relight but still not working. can anyone help?