How is it caused?
Radiators may not heat up if; there is air trapped inside them, the radiator valve is faulty or the radiator has a build up of sludge within it. Radiator valves are less likely to function as they were once designed to due to their age.
If you can feel cold patches at the bottom, centre of the radiator this is likely to be sludge in the system (magnetite), that has collected in the radiator.
What should I do?
If your radiator is hot at the bottom and cold at the top, it will likely need to be vented of air. If you do this, be sure to refill the system pressure if required, using the filling loop.
Any other problems with radiators not heating up will need to be looked at by an engineer. They can identify the correct course of action for your problem, which may include the need to have you central heating system powerflushed.