COOKER HOOD switched off during operation?

I have recently installed (professionally) a BAUMATIC BE605BGL cooker hood. As soon as I put more than one saucepan on to the hob the hood stops working. The lights remain on and the "fan" continues to be displyed (on the illuminated panel) as circulating - but the hood itself does not extract.
I have checked that it has been installed far enough from the hob (as suggested in the instruction manual) - but this problem persists!
Can anybody give me any advise as I have only read good reviews about this product and do not wish to call Baumatic until I am sure that I have not missed anything obvious?