Problems with one radiator?

I have read the advised given to another person regarding ONE radiator not getting warm, and I still have a problem so looking for some help.

I have closed off all the radiators bar the one that has a problem. Have bled it, and have left the bleed valve open - the intake pipe is warm but the radiator is still cold - no sign of any heat. I have a thermostat valve on the other end it is fully open and the pin is not jammed. I have tapped the radiator and the thermostat valve in case there is a blockage but no joy.

One point is when I open all radiators and open the bleed valve on this radiator the intake pipe does not get warm - it only warms when other rads are off.

Any advise please