Hi, there could be an air lock in the new rad/pipework. Make sure its bled . Also the pump might be struggling to pump down for the extra radiator.
I would turn all the other radiators off . This would force the flow to that radiator p. It may push through any air lock and if it gets hot prove that the valves are working properly.
If it gets hot, slowly open up the other radiators bit by bit till they start getting warm too. The radiators closest to the pump should be opened only slightly. The ones furthest away fully. This is balancing the system so the flow is evenly distributed to all radiators