Siemens RDH 10 RF Thermostat not triggering heating?

I have Siemens RDH 10 RF Thermostat and a Worcester Bosch Greenstar 30CDi combi boiler. The battery in the thermostat is fine. The green light is on on the wall sensor. When I turn up the thermostat to demand heat, the orange light flashes on the receiver, but the boiler doesn't fire.
The boiler does fire for hot water. Any advice please?
Thank you