I had problems with mothers house, in her case a Bosch boiler, but the problem was return water too hot, and this auto turned down the boiler. It seems the idea is the TRV controls the temperature of each room, and once all rooms are warn the TRV's will all be closed so the by-pass valve lifts and hot water returns to boiler first turning the output down, and then it starts to cycle off/on.
However if any lock shield valve is wide open, that will also cause hot water to return and the boiler with turn down, so what was happening was it was heating just one room at a time, once the room was warm the TRV would close and send warm water to next room.
So the setting of the lock shield valve is very important, it seems one should measure water temperature in and water temperature out and there should with TRV wide open be around 15°C between the too readings, I tried but the thermometer idea seems good in theory but in practice could not get a steady enough reading.
So I turned off each lock shield valve in turn starting closest to boiler, allowed pipes to cool, then opened ¼ turn at a time until I felt a little warmth on the feed pipe, then went to next.
Returning to first radiator it was now quite hot, but some were still cool, so had to tweak each one ¼ turn more or less. This was much improved, but not spot on, to try and get it better I got some electronic TRV heads, these have the target and current temperature displayed
so I further trimmed the lock shield if after running for some time target was under current I would close a tad, and if other way open a bit, the problem was old TRV heads were marked *123456 which is about as much good as chocolate fire guard, once I knew the TRV was set correct, then only one thing to set, the lock shield, I actually moved the electronic head from radiator to radiator, once set the old mechanical was OK, around 2.5 = 20°C.
Once all were set, when boiler first fired up the water was hotter, and all radiators heated together, I was really pleased with how the TRV heads worked.