In many cases, radiators are heating unevenly or losing heat too quickly. Poor circulation or imbalance can leave rooms under-heated even though the boiler keeps running.
Heat loss also plays a major role. Draughts, cold walls, and uninsulated areas can undo the benefit of continuous heating. That wider issue is explained here: Why Your House Loses Heat Quickly.
Another factor is thermostat placement. If the thermostat is satisfied before colder rooms warm up, the boiler may cycle inefficiently without delivering comfort where it’s needed.
If radiators also cool down quickly once the heating stops, this pattern overlaps with: Heat Escapes Fast After Heating Turns Off.
Understanding how boiler runtime, radiator performance, and heat retention interact is covered fully here: How to Keep a UK Home Warm for Cheap.
To pinpoint whether the issue is boiler-side or building-side, use the diagnostic tool here: House Cold Diagnostic Tool.