Some heads are better than others well maybe different would be a better word. There is a range at which they work at so set to 20°C mine (Energenie) work flat out until 18°C then they start to close, so to get from 18°C to 20°C can take 2 hours.
So I cheated, at 7 am set to 22°C then at 7:30 am set to 20°C so by 8 am it was spot on. OK these were programmable but if anything the cheap wax type are worse and start to close even sooner.
So simple test, remove the TRV head, this means that radiator is running flat out, does the room hit the temperature then? If so then problem is head, if not then first move is to open the lock shield valve a little more.
If house set to 20°C then with TRV head removed would expect at least 23°C may be more, I set all mine with two sensors one on feed the other on return so there was a 15°C drop in temperature between the two. However this is a start point, not final setting, if the room over shoots close a little, if not reaching temperature open a little.
The problem is as one adjusts one room, this alters other rooms.
What would really be handy when setting up is if some one said at X°C water return temperature the boiler starts to modulate. Or a gauge on the boiler 10% to 100% output. If you know boiler output is 100% then you also know nothing you can do will get rooms warmer, if at 50% then maybe radiators can be opened up a little.
I am an electrician not a heating engineer, but if some one said once return water hits 60°C the boiler starts to modulate one would know what to aim for.