One radiator in a bedroom is always cold?

One radiator in a bedroom is always cold.
It used to work fine. I have tested the thermostatic value and the pin is not stuck. I have tested with the head of the thermostat unscrewed (which means fully open/uncontrolled).
I have bled it to confirm no air is the cause.
I have tried to balance the radiators and I think this is likely to be the cause. I have good hot water in a pressurised hot water system. Flow to the furthest radiator from the boiler is good.
What tricks are there to test and get a flow to this problem radiator? I want to try something even at the expense of heating in other bedrooms.

G Smith
February 2026