One Cold Radiator

John S._2

My home has two floors. I have circulating hot water with 4 zones. One zone on the 2nd floor serves 4 radiators. One of the four, the one farthest away from the boiler is much colder than the others (it is a corner room with 2 outside walls). It has a large cast iron radiator, the room next door has the same (somewhat smaller) and the other two rooms have Sunrad type newer radiators. I have bled it and it does get hot if I crank the heat way up. Otherwise it never gets warm in the room before the thermostat is satisfied.
Are there any suggestions to improve the flow to that device besides creating a single zone for it? Any help would be appreciated.