This could be due to what is known as an “airlock,” which can be a reoccurring issue with many radiators. Anytime water is added to the heating system air will enter the system as well. As air is lighter than water, the air will settle within the radiator. This means less water running through the central heating system and colder room temperatures, not to mention an increase in your heating bill. A fantastic way to tell is to feel the radiator itself: if it’s colder at the top than at the bottom, there is a good chance of airlock.
While bleeding a radiator is common practice among homeowners, you can sometimes find yourself in the position of having bled the radiator but it still continues to remain cold. We never recommend any DIY or “cowboy” plumbing jobs when it comes your home’s central heating, so it would be best to call a Gas Safe certified plumbing and heating engineer to inspect your radiators.