Usually the problem is air in the radiator. if they are hot water then there is an air bleeder at the TOP on the end of the radiator. If you have a steam system the air vent is 3/4 " dia. x 2 1/2 " long and conical in shape, while there is steam pressure on the system, lightly tap on the vent and see if it starts to work. If you have a 2 pipe steam system, the steam trap at the opposite end from the valve could be defective, or the radiator valve may be the problem.