westherm thermostat radiator valves

confused1

I have the above fittted in my radiators.
Lately, some of the radiators have stayed cold or just lukewarm, inspite of turning up the thermostat valves. I have bled the radiators, but that still gave no joy.
Maybe the valves are 'stuck' from infrequent use. If so, how do I go about releasing them?
Any advice would be very much appreciated.
Thank you

Cardew

This happens a lot with my infrequently used TRVs(not westherm)

If you unscrew the top of the TRVs you will find a 'plunger' is probably stuck down. Just gently pull this up with a pair of pliers and 'exercise it a little.