What I know about steam heat fits easily into a thimble so maybe I shouldn't ask this but here goes.
If the vent on that radiator is working correctly then is it possible that the return line is just plain clogged somehow or that it doesn't pitch the way it needs to?
I know those radiators aren't easy to remove but if you did remove it you might be able to see what's going wrong there. Provided your sure everything else is working as intended.
BUT - if the real answer is NO stupid, this is what it is, then that's OK with me because I don't know anything about steam heat but would learn something along with you.