I have the exact sme problem, I think.
You have to be careful with the internet. Lots of people with good intention but info provided is wrong. Often, because they did not have sufficient info to do the diagnostic.
Here goes.... all my radiators works fine except one. The system is a SINGLE PIPE STEAM HEAT. If your system is not single pipe steam heat, please ignore the rest.
The steam goes to the radiator with one pipe and the condensed water goes back the SAME pipe.
There is suppose to be air in the system
There is no need to bleed anything.
There is no traps or pumps.
I doubt there is any clogs.
Steam heat in my childhood house, decades of operation, never needed a "cleaning o fthe pipes" or a clog.
My 100 year old house with steam heat system. I doubt anyone ever cleaned anything. The 20 years that I own the house, I never clean any pipes.
The ONE radiaor that gets filled with water ... I remove the vent, water spills out. I open the pipe, unattaching the radiator from the pipe, water spills out of the radiator. Clogged pipe? NO! NO! NO! I poured a gallon of water into the open end of the shut off valve, with a funnel, and the water goes down without problem. My son can hear the water made it's way to the boiler.
Check the pitch of the pipe and the radiator. I did that for my case, did not fix it. One OTHER radiator had the same problem when I first bought the house. Correcting the radiator pitch and tighten the pipe bracket correcting the pipe pitch fixed the problem. I still have to figure out why THIS radiator still have problem.
I have changed the valve. Easy to do. It can be stuck, by rust, or painted shut at the opening. The valve is open for air to go BOTH ways. It shuts off as soon as it senses steam. As the boiler turns on and produces steam, the steam is pushed to the radiator. The air inside will block the steam if it has nowhere to go. The vent takes care of that.
Steams cools and turns to water and drains back the same pipe. As it does, if no steam, the vent is open allowing air to come back in the radiator. Cycle repeats.
So, for you, 3 things to check. Vent, radiator pitch, pipe pitch. It worked for one of my problem radiators. The remaining problem one was altered. It was moved to a new location so the pipes were extended. There may be other issues.
Good Luck.
I welcome any advice. Knowledge of steam heat is a lost art.