A common cause for radiators being cold at the bottom but hot at the top is due to a build-up which reduces the flow of water around the radiator.
This build-up is now a combination of limescale and dirty water is now circulating around your heating system.
Eventually the combination settles at the bottom of the radiator to form ‘sludge’, restricting the flow of hot water around the radiator leaving some areas and the bottom of the radiator cold.
Radiators with a build-up of sludge can make heating your home difficult, increase the cost of your energy bills and even cause damage to the rest of your central heating system.