Remove the radiator, take a bucket or large pot and hold it under/in front of each rad valve and open it up again slowly, see if you get a good pressure and constant flow out of it. I'd suspect there's a restriction in the pipe. A flush machine could loosen it but at the same time BG does note that 10 and 8mm pipes will be harder, or impossible to flush if their blocked. Don't forget tho that BG will cover reflushes of the original Powerflush
Personally I use a good strength rubble sack, obviously one without holes, as opposed to a bucket/pot. It holds more water and you have more flexibility with positioning it under the valve. You can actually pull valve away from radiator and pull bag through the gap and radiator will drain straight into it.
thats the way I would go as well, remove the rad then connect a hose to each pipe/valve and see what comes out, if nothing connect on hose to the mains and see if it will push the other way.
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