Boiler needs kick started by hot water for heating?

We have a Combi boiler controlled by a Hive thermostat. The scenario is that you get up in the morning and the house is cold after the time the heating was due to come on. You turn on any hot water tap in the house and the heating then starts operating. Why does it not come on when it's supposed to in the first place? So far as I can see the boiler is getting signals from Hive and the house is still cold long after heating was supposed to be on. For the purposes of suitable responses, assume I know how to use a screwdriver...