Sime super 90 burner problem?

if you turn on the hot tap on the pilot light clicks burner fires etc all works as it should hot water central heating etc,BUT only with the burner chamber door open a fraction 1/2"-1" if you close the door the flame is extinguished and will not fire up again until the door is opened again.any ideas anyone? I have checked the ducting and fan (seems to be drawing good air flow although it is a bit noisy dry bearing kind of sound) Many thanks for any pointers..