you may have already sorted the problem,but if not I suspect as the system is new and the rest of the rads are fine it could be the TRV on that rad which is faulty,you could try and take the top off the TRV you should see a needle sticking up it may be stuck try to move it up and down do not use a lot of muscle power if it is stuck spray some plus gas on it try a pair of pliers on it do not bend it,if you can free it fine if not get a new valve.
Best Regards