All except one radiator are working fine. The one that stays cold is in the room with the thermostat, so the system keeps running more than it should.
I tried bleeding it and just water came out. I let it bleed for some time and the pipe which comes down from the ceiling did slowly warm up, but the radiator never got warm the next day the pipe was cold again. The pump was replaced a year or 2 ago and seems to be working fine.
It worked last year. Could there be some sort of blockage? If so, what can I do about it? We are in a hard water area.