This could be due to a few things, air locks, blocked or restricted pipework, system out of balance or faulty valves. Try the following: 1) Close all radiators Thermostatic Radiator Valves (TRV’s) except on one of the problem radiators. If the radiator then gets hot this will rule out a problem with the valves or a blockage. Repeat with the other problem radiators. 2) If the radiators worked then it is either an air lock which is moving around the system or it is a problem with balancing. If it is an air lock with a little persistence you should be able to eventually clear it by bleeding the r