It may be a fault with timeclock/programmer, the output from which gives a supply to room 'stat, which then sends a supply (when calling for heat) to valve, this in turn sends a demand signal to boiler...
If you have means of testing for live at 'stat (not a neon test screwdriver),
and it is live with time clock on, you should be able to check out going sw.live ON/OFF as turning up & down.
Alternatively switch off boiler supply, at 'stat temp. join live in to switch live out with a connector, leave Neutral as is.
Turn on supply and timer should move valve and fire boiler by selecting On/Off. If boiler fires 'stat needs replacing, if not your problem is either timer or valve.
There seem to be a lot of similar posts at the moment due to first seasonal use of heating systems, I have only had 2 faulty 'stats in last 10 years or so, there is very little to go wrong unless digital type which won't 'click' if no power to them..
Usual fault is programmer loose on base due to decorating etc. if plug on type,
If you are not happy with testing at mains voltages or don't have a decent meter please get leckie in,
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