I also have the same problem with one radiator on the top floor of the house. The inlet and outlet valves are fully open, and the bottom few inches of the radiator get scalding hot after just a few seconds of the water starting to flow. I don't have a sealed system, the header tank in the loft is full, and all other radiators on both floors are piping hot at the top and bottom. There is no system bleed valve that I can find in the loft. Instead, I seem to have an expansion pipe that simply vents back in to the top of the header tank, above the water line.
The radiator in question is a 'half height' radiator, with the result that the bleed valve for every other radiator on the same floor is physically higher from the floor. Because none of these radiators have any air in them, I therefore know that the head of water in every such radiator is higher than for the radiator in question. With a full header tank, I am therefore at a loss why this radiator won't fill, and would appreciate advice, please.
Note that having had the problem, I removed the radiator and flushed it through with a hosepipe to check it isn't blocked. It isn't. I then switched off (closed the inlet valve) every other radiator in the house. This also made no difference.
Many thanks for any help given.
This is driving me nuts!