? valve faulty

I drained my open vented ch system 3 weeks ago after having X400 going round for a month to try and sort out a partial blockage. I flushed the system out well and on advice, as the drained water was the colour of oxtail soup, put another dose of X400 in again. After filling the system I was left with a cold rad in the kitchen, did all the usual things to try and cure it with no luck. I even drained off the rad again. Water flowed ok from the inlet side but only a cupful from the return side so assumed a blockage somewhere and decided to leave any further investigations until I drained off the system again.
Today I drained just the rad again, this time water flowed from the return side but only a medium trickle. I took the rad off opened return valve again fully, again same rate of flow. Forgot to say that after emptying rad I filled it again and opened valves to see if it worked, no luck, had closed off all other rads again no luck.
With water trickling out of valve I shone a torch in the opening and to me it didn't seem to be open enough on full opening of valve nut. I think they do not look fully open anyway but this seems to be opening only a small bit. Is this a possible cause of the cold rad? I am going to drain the whole system again within the next few days so am leaving rad off.
Anyone any ideas?

if you suspect the valve is not opening change it for what they cost.

Thanks, I was thinking of doing that as I need one with a drain off point, but could valve have been the cause of the cold rad even though it was letting some water through?

did you try shutting the other rad valves and leaving just the kitchen one open ?

Yes tried that already, a few times, no luck.