New Glowworm 24c betacom- not heating radiators?

We had a brand new Glowworm 24c beta com combi boiler installed last week, with a digital thermostat in the living room to control the central heating. It worked fine with no problems for around 7 days, but now its started to not heat the radiators on 2 out of the 3 timer 'zones' on the thermostat.
The boiler switches on but doesn't seem to fire up and heat the radiators. However, if we then go to the boiler and flick the switch for hot water/central heating or just hot water from the normal position of hot water and central heating off momentarily, and then back to the same position, the boiler then fires up and it works fine. The hot water is fine as always.
It only seems to be affecting the morning and night programmes on the timer, and the afternoon one seems ok.
Anyone got any ideas what this could be? Its very frustrating....!