Outlet side of my cast iron radiator is cold. What's wrong?
Given "I drained the whole system" I'd suspect a trapped air bubble first and foremost. Try bleeding again. The bubble may be trapped in the pipe rather than the radiator itself - look for bleeders along the pipes. Alternatively there may be a valve you've missed, or an actual bit of debris plugging the radiator (a pretty distant third place, in my experience.)