vokera excel 80 E combi does not give hot water in DHW mode?

It gives hot water in the taps only when CH mode. Some times in DHW mode it attempts to fire and the fan may come on and ignites but other times the fan does not even come , Also the pump goes on and off as it attempts to fire. Pleaser advise. Surelt the dhw flow switch and the dhw micro switch must be ok as thse would need to be working when you get hot water at taps in ch mode.