We have no heating on certain days ?

We have no heat on certain days sunday is one of them. The green light is on the main control and is set to on all the time due to the bad weather. We have tried re setting the whole system to the factory times on the digistat box. I dont think the main controls are communicating with the digistat box the batteries have been changed. I think this is all to do with settings as some days we have heat and the radiators come on. As any body got any ideas this is a very confusing complex system!!