On Tuesday, December 18, 2018 at 6:20:31 PM UTC-5, wrote:
A friend has an outdoor wood furnace going to a building. He put two
radiators (the old cast iron type) in the lower level and they heat
well. He put another radiator in the upper floor and that one will not
fill up. It's only heating the bottom of that radiator. I opened the
bleeder while it was running, and no air or water comes out. I then
removed the entire bleeder and still nothing comes out. There does not
appear to be a clog behind that bleeder.

I am thinking one of two possibilities. Either the pump on the furnace
does not have enough pressure to lift the water to that second level, or
else that radistor needs some sort of restriction on it's OUTPUT side to
force the water to the top of the radiator. Currently he just has both
pipes going to the radiator, one to the bottom left, the other to the
bottom right.

(This is a HOT WATER, -NOT- a Steam heat setup).

Anyone have any suggestions?

Any of the other radiators have valves or thermostat valves? If so close them and see what happens. How about filling it up using the air bleed hole? Could be pump head capacity, size of pipes, length of run.