Radiator warm at the bottom but cold at the top
This is usually due to air in the system. Most commonly happens after you've had the heating off for lengthy periods.
Use a radiator key to "bleed" the air out. Unscrew the bleed screw just enough so you hear air hiss out. Get ready to tighten it up as soon as water starts to come out. The radiator should feel warm at the top now.
N.B. If your heating is a combi boiler or high pressure system then bleeding will reduce the pressure and may stop them working. You will need to repressurise the system.
(see Low pressure on combi boiler)