Can you give us some more information on your set up?
Do you have an extension kit controlled hot water too? If so, does that work? and do you hear the relays click when you turn on/off the heating or hot water?
If you don't have an extension kit, does your smart thermostat relay click when the heating is turned on/off?
I have previously had the issue where my extension kit seemed to lock up. Powering it off for 10 seconds and back on resolved it