How can I fix Potterton Profile 60E boiler?

Potterton (Baxi) Profile 60E. Air pressure switch never operates, though I can cause it to flip over by blowing into the high-pressure tube or sucking the low pressure one, at the remote ends of the tubing. Have I a faulty switch, or just what is it that causes the switch to operate? So far as I can see, both the high and low pressure tubes seem to go into the output flue.