help - 1 cold radiator?

I have 1 radiator that wont and never has heated up. The system is balanced and all other rads work fine. the lockshield valve is fully open. the pin on the trv is not stuck but the rad and both pipes remain stone cold.
I recently, out of desperation, tried completely removing the bleed screw, while the heating was running. After a short while the radiator got nice and hot, as did both pipes. this led me to believe there were no blockages in either the supply or return pipe. As soon as the bleed screw was replaced the rad and pipes started cooling and are now stoneycold again.
anybody got any ideas??

cheers in advance