This often happens when the boiler flow temperature is set correctly but circulation is restricted elsewhere. Balancing issues, partially closed valves, or pressure problems can all limit how much heat reaches the radiators.
Low pressure is a frequent contributor. Even a small drop can reduce circulation enough to leave radiators lukewarm rather than properly hot. That pattern is explained further here: Boiler Pressure Drops Overnight.
Radiators that take a long time to heat or fade during the cycle are another clue. That behaviour is covered in detail here: Radiator Takes Ages to Heat Up.
In many homes, this situation improves dramatically once the system is balanced properly. When flow is shared evenly, radiators reach temperature faster and stay warm longer.
How radiator performance fits into overall home warmth is explained clearly in the main guide here: How to Keep a UK Home Warm for Cheap.
If you’re unsure whether the issue starts at the boiler or further along the system, this diagnostic tool helps clarify it: House Cold Diagnostic Tool.