I have a couple of rads that are sluggish. The radiators are clean and I have found that there is a good flow of water out of the return tail - but a very sluggish flow out of the Flow tail when things are opened up.
Is it possible that the manifold could be sludged up? Alternatively I am thinking a pipe may have become crushed somehow. It was once working well. The system always has Fernox in it. I have even tried blasting mains water back up the flow pipe in case there was an air lock.
As it is 2 / 3 radiators I am hoping to find a common cause.