Is the lockshield valve open?
Have you vented the radiator(s)
I have vented the radiator & presumably the lockshield is the manual valve at the other end of the radiator - these are open.
Looks like it could be a plumber job !!
I have Lifestyle thermostatic valves fitted to my radiators, three of the valves do not appear to be allowing hot water into the radiator, which is obviously staying cold. I have removed the top white body & also the plastic surround around the pin. The pin moves up & down against spring...