Cold radiator with cold water on circuit with hot radiators

Providing the pipework has been installed correctly to flow and return with orientation of directional valves unless of course the valves are bidirectional try shutting down the valves approximately half way on one end of each of your other radiators in the circuit, this will reduce flow to your other radiators balancing the system. A couple of factors to consider is the size of pipework used to the new radiator and the length of run from the existing circuit. When you have achieved a satisfactory flow to the new radiator work back on each existing radiator opening the valves you have shut down half way by an additional quarter of a turn to each, check the new radiator is still receiving adequate flow and repeat the process opening by the additional quarter turn until all radiators are receiving an adequate flow of hot water. I hope this helps.

It is very possible that something is stopping the water getting through one of the valves. The first thing I would check is that both valves are definitely on and if you have a thermostatic radiator valve I would unscrew the top and make sure the pin is not stuck as this is very common. I hope this helps

Most likely the pipes will be mixed up for this radiator. In/Out.

See this alot . Sounds like a stuck valve try .opening and closing valve a few times or giv a little tap . See of the heat moves into the radiator.if not it will need the valves removed to inspect .the problem is flow .(it's not) only possiblity is the valves👍