Why can't I get hot water for the shower from my Vaillant boiler?

I have a Vaillant turbomax VUW 242E/282E (not sure which) and I just had a re-occurence of a problem in getting hot water to the shower, which is a mixer fixture attached to the bath taps. Central heating is fine and can get hot water via the taps of the bath and sink but turning on the shower does not ignite the boiler and pressure (i.e. flow) is lower than it should be. Boiler pressure is fine. We had a leaking diverter valve, which resulted in the same problem, replaced three or four months ago but this time there is no sign of any leaks. Is it just likely that this replacement has failed or is there any other possible explanation?