Baxi Pw5 wont fire up automatically!

[msrxkah4]

Hi, I have a baxi back boiler (I think its a bermuda PW5). When we turn it on, sometimes it does not fire up. However, if you twiddle the knob from high to low under the fire place, it starts to click and then fires up. (The red light goes yellow). Would anyone know what the problem is.

Thanks