Why your radiator is cold at the top
If you have a cold patch at the top of your radiator, this always means you have trapped air within the system.
To fix the issue, you'll have to bleed your radiator so any trapped air can be released. Do this for any and every radiator that has patches of cold at the top of the radiator.
Why your radiator is cold in the middle
If your radiator has a cold patch in the middle, then there could be a build-up of rust, debris or sludge at bottom of your radiator that's getting in the way. This can be fixed by cleaning your radiator out entirely.
If you have an open-vent system (usually a regular boiler) you'll be able to use a sludge remover to flush your radiators. If you have a pressurised system (usually a combi boiler) you'll need to get a Gas Safe engineer to fix the issue.
Why your radiator is cold at the bottom
If your radiator has cold patches at the bottom but is hot at the top, then you'll likely have a build-up of limescale, rust and sludge within the radiator. A mixture of these waste materials can cause a blockage and obstruct the flow of water.
To fix this issue you'll need to flush every radiator that has a cold bottom, this should push out the mixture of build-up and restore the flow of water. This will need to be handled by a Gas Safe Registered Engineer with the correct equipment.