Any idea whats wrong with my Worcester Bosch 24i Combi?

We have a fault that when we turn the hot tap on we do not always get hot water, the boiler does not ignite, turn the tap off and it may ignite, not always and then turns itself off. We may use the hot water for a few hours whithout any problem, say all morning, go back to do the lunch washing up and it will not ignite. We have had a number of plumbers look at this over the years but they have never cured it