Cold Add-on Radiator?

My heating system has a single zone. At some point, a previous owner added a radiator in the basement. This radiator is always cold, although the rest of the radiators seem to be working fine and the pressure and temperature at the furnace are perfect. I tried bleeding the radiator, and got only cold water. However, if I continue to bleed the inlet pipe stays cold but the return line warms up. The only difference between this radiator and the rest of the radiators is that this one is below the level of the furnace while the other ones are above. Any ideas of what is happening or where I can begin to troubleshoot this thing?