If the middle of your radiator is cold, there may be a build-up of debris or sludge which is obstructing parts of the bottom of the radiator. You’ll need to clean it out and remove all unwanted substances that are blocking the bottom length of the radiator.
For all pressurised systems, you’ll need the assistance of a Gas Safe registered engineer to work with your system to pressure flush your system and radiators from sludge and debris built up over time.