If the thermostat is still calling for heat, the green light on the receiver should stay on, so it would appear that something is amiss.
There may be a communication problem, is there something between the two units that blocks the RF signal? for example a new fridge freezer? or has the room thermostat been moved further away from the receiver?
I have come across two instances where there was the same device in a neighbouring property that switched the device next door on or off. The problem is that all RT500RF's leave the factory with the same RF settings, and few installers bother to change it. If that is the case manually changing the RF code to a different setting will resolve it. The instructions
tell you how on pages 9 to 11. But make sure you follow them to the letter