boiler lights, but just ticks over, IE: it won't go to full flame?

The boiler is a Radiant RS24. It lights, but stays on tickover. We've had a new pcb, limit thermostat and heating sensor fitted and it is still the same.

Note: Before we replaced parts, if we shook the control panel with the PCB in, the flame rose to full.

Any ideas as to what's causing the problem?