Single radiator luke warm top cold everywhere else (not sludge up)

[Scotty001]

Hope you can stay with me on this a little long winded but thought as much info as possible as it may help.
Some history
A) 8mm microbore system on a combi boiler
B) single 22mm to 8mm manifold
C) 10 radiators on the system (3 are 600x600 type 22, 3 are 600x1100 type 21, 2 600x600 type 21. 1 600x1100 type 22, 1 towel radiator in the bathroom
C) Have had issues with blockages in the past
1) many years ago a section of pipe (rectified)
2) blocked heat exchanger in the boiler, (that part was replaced last year with a adey micro filter system installed to catch any more metal fragments along with a good flush of the system)

The issue now
The radiator in the front room only gets slightly warm on at the top, one valve (lockshield) and pipe gets boiling hot, the other side (pegler bulldog trv) and pipe stays cold.
Yesterday I replaced the radiator with a brand new one as figured the older one had sludge up, but I still have the same issue.

Today I took the radiator back off the wall, opened the trv (side that always remains cold) to drain the pipework and warm water was coming out, I also opened the lockshield and warm water came out of that too.
I replaced the trv valve and head with a pegler terrier 3 (bi Di rectional) to rule out an issue with the valve, put the rad back on the wall, re pressurised / filled the rad up (from the trv side)

With all rads bled and boiler back to preasure I opened the lockshield to full open and the trv set to 5 and turned the heating on. 2 hrs later all radiators were hot but this front room radiator still only slightly warm at the top and cold at the middle and bottom, the lockshield and pipe is very hot but the trv valve and pipe is still stone cold.

I'm at a loss of what to do now?

[aeromech3]

With 8mm it is very easy to get a kink in the pipes, can you see either side or does it bury in concrete?
Have you tried with nearby rads closed off?
Do you have a variable speed pump if so what setting?

[Razor]

Is the manifold upstairs with the living room rad on a drop?

Turn all the other rads off and run it with just the living room. It’s probably an airlock