Trapped air in a radiator will prevent it from working correctly. You can tell if there is trapped air by placing your hand on the radiator. If it is cool on top and warm at the bottom then this is a sign that you will need to ‘bleed’ the radiator (remove the trapped air).
All radiators have a bleed valve on one of the top corners.
You should switch off the boiler and wait for the pump to stop, then place a rag or jug under the bleed valve and using a bleed key, turn the valve anti-clockwise ½ turn.
When you do this, you will hear a hissing sound as the trapped air releases. BE careful when you do this because the water can be very hot!.
As you release the trapped air, you should be able to feel the heat rising within the radiator by placing your hand on the top of the radiator. Get ready to close the valve as soon as water starts to come out of it.