I have a similar problem. All my radiators work brilliantly, except for one. I have bled it and that's not the answer. It is full of water, only the water is cold. The feed is hot, and the valve is open, but for some reason hot water doesn't seem to be going into the cold radiator. It was switched off for many years, and since I turned it back on has never got hot. Grateful for any suggestions.
the lockshield valve was indeed the problem - turned out it was turned off. So recommend looking at that for the other person who was having a similar problem - I just needed to unscrew the plastic cap and give the valve a couple of turns open with a wrench / pliers, and problem sorted! Many thanks to the person who suggested this.