Need some advise from a experienced master plumber please.

Cold weather has arrived so fired up the central heating and the double radiator in the master bedroom is just getting luke warm from the top, bottom is cold.
The boiler, rads, valves & piping is all new & only one year old. The whole system is new & was working fine when installed. Yes, power flush was done & 2 bottles of Fernox inhibitor installed.
Not needed heating but used boiler for hot water and no problem. The Worcester boiler itself is working fine.

I have 11 rads in total & all seem to work fine except the double rad in my bedroom, which just stays luke warm.
The incoming water pipe into the rad is hot but the rad remains cold or just luke warm.
I have blead the rad several times and no air is trapped.
I have removed the cover from the rad TVR & the pin is not stuck, it seem to be fine. The valves are open and rad is full of water.
I have put into the system 3 bottles of Fernox F3 system cleaner for over 2 weeks to clean the whole system. Still no luck.
Today, I drained the whole system, removed the double rad & flushed it with a hose pipe for a long time.(only a little bit of black sludge came out)

Put rad back & re-filled the system, blead all the rads and the same results.
All the rads warm up properly, except the one in my bedroom.

The water pipe that supplies the rad with water is hot but rad is cold/luke warm. What does that mean & what could be the problem???

Any ideas what could be the problem & what I should try doing next???

Thanks