Radiators not staying hot

[Ineedalifeline]

Just after some central heating/radiator advice.
Replaced 6 out of 8 radiators total in the house over the summer, including a towel rail in the bathroom.
Come winter, only 2 of the radiators seem to be working well, the other 4 come on as programmed and get hot but then go off a long time before the programmed off time. The boiler stays on (Worcester Bosch, about 2 years old) and the hot water (tank based) is working well. The towel rail does not heat up at all, all pipes connected to it are stone cold, all radiators have been bled so no trapped air.
Would love any pointers as to what is wrong whilst trying to get the heating engineer out to fix. Thanks in advance.

Who changed the radiators? Has the system been balanced?

turn off the hot ones. Do the cold ones now heat up? If only some, turn off the hottest.

As Goosey has in mind, a competent person would balance the system after fitting new radiators (or valves)

Where is your room thermostat? Is it in one of the rooms with the hottest radiators?

Balancing will fix that. For a quick attempt, after running the tests above, you can turn them all off at the lockshield valve, then open each lockshield half a turn ONLY from the closed position.

Check that each radiator is working individually. Turn all the radiators off and then turn each one on at a time. If they all get hot this way in turn, then chances are your system just needs balancing. If you have particular radiators that don't get hot at all, then you have a different issue (and I would call back the people who installed the radiators cause chances are they've fucked up). Oh and make sure the lockshield is open when you test each rad.

You should also check what pressure your heating system is at. If the pressure is low or keeps dropping then you have a leak. I'm sure pigletjohn will be along with more thorough advice.