vokera 80 e combi boiler no hot water?

The boiler does not ignite when the hot water taps are opened,

it does not attempt to go through the motions of trying to
ignite. There is no sound from the boiler. The green light is on
so the boiler has not gone to lockout. I can only get limited hot
water when boiler is in central heating mode and the burners are burning. When heating is up to temp and boiler goes to rest
mode, there is no hot water. What can be causing this?