All the radiators work except one. I have tried a few things but still?

Hi, Hope you guys can help.

This winter my radiator in my bedroom doesn't heat it up properly, it is warm at the bottom.

I initially bled it and as we have a combi boiler, after I had to bring the pressure up by opening a valve near the boiler. But that didn't work it remained cold.

I then switched off all the radiators in my house except the faulty one in my bedroom. (However the one in the bathroom didn't switch off properly and remained hot so not sure if this affected it). I then bled it again a bit. After this I think the bedroom radiator did get a bit warmer (the warmth spread), but still the whole unit does not get hot.

The water comes out a bit dirty and smells. Could it be rust in the radiator stopping the water spreading properly in the radiator? Help. Is there anything else I can do? Or do I need a plumber to flush it out?

Thank you so mch