Cold Radiator?

The thermostatic valve could be the cause.

Try setting it for max heat.

If that makes no odds, remove the top (usually unscrew the ring underneath the head and lift it away from the body).

There should be a pin sticking up from the body, which the thermo head pushes down to turn off the radiator when it's hot enough. You could check that is free to move (it does have a fairly stiff spring underneath - you won't be able to move it with your fingertip. Use a suitable blunt object to avoid getting a hole on your finger. A small squirt of WD40 might help.

There should be plenty of YouTube videos about removing / installing TRV heads and freeing them.

Alf
September 2022