glow worm 50?

i have glowworm micron 50works ok when it calls for hot water but will not always fire up for central heating can you help have wirless room stat which re programs ok and says calling for heat when boiler does fire up will run till reaches temp ok but then will not restart it seems to want tobe cold before it will run again it did have new pcb fitted but that was about three years ago they had a fault with them on the origanal ones but glow would not admit it thanks pete