Are Your Radiators Corroding/Leaking?
This may seem like an obvious one, albeit an irritating reason! But unfortunately, steel does have the potential to fail and leaks can form. If your radiator is in warranty, it will usually be covered and able to be repaired/replaced, depending on what is outlined in the guarantee.
In regards to corrosion, as water runs through the system, it is inevitable that corrosion will become a problem. It is important to keep your system regularly checked. If it is left unchecked, the corrosion can build up and cause sludge that will block the water resulting in insufficient heat output.