Cold steam radiator

I have one radiator (single pipe, steam) that is cold.

Back story: I had a few cold radiators on both floors of my house. I put new steam valves on them and they all worked again. A couple weeks later, one of the rads I had fixed stopped working on the second floor. I bought a new steam valve again. Still cold. Even the pipe to it was cold.

I turned off all the other rads in the house and the problem rad did get hot, but it took a very long time (more than half an hour, whereas they usually heat up in a minute). And even then, only the portion of the radio closest to the pipe got hot, the rest was only warm or even cool.

Any suggestions?