Radiator not getting hot

Peter King

I know next to nothing about his stuff, so here's my dumb question. I've got a one-pipe steam system in a three-story house and one of the radiators on the 2nd floor, coming off a riser near the end of the main, only heats up the first two fins. I raised the far end up to make sure condensate was draining. I put a new air vent on. I even swapped the whole radiator out with one I know works. Same thing: only the first two fins get steam. The air vent sounds as if it is sucking air in and out and there's some splattering. There is only one radiator further done the line, which heats up fine but the vent sprays like all get out. Any ideas?