Have they got Thermostatic valves on them? And how old are they?
These can be removed from the radiator without the water pouring out! If the radiator then becomes hot then it is a faulty thermostatic valve. If it still doesn't come hot then get a pair of pliers and pull up the thin pin that was exposed from removing the valve. The pins can get stuck down. I've just had this done on my radiators and they are now working great!
Ric
December 2008