Back.
So it appears that the system needs balancing. Other people reading this might have different causes.
Your objective is to slow down the flow of water so that it travels through all the radiators at about the same speed. If it is unrestricted, it will rush through the easiest path, and hardly at all through the longest or most difficult route, resulting in some hot radiators and some cold ones.
The valve at the opposite end from your TRV or knob is called a "lockshield". This is because the spindle is covered by a plastic cover which is fixed into place, usually with a screw. It does not have a turnable knob. This is so that once you have spent time and effort adjusting it, nobody will come along and turn it. The flow through any one radiator will affect all the others, so you have to do them all. It is useful to screw down each lockshield to fully closed, and write down the turns, to the nearest quarter-turn, so that you can put it back to its previous adjustment.
It is essential to know that water does not squirt through radiator valves like through a sink tap. It moves quite sedately. Also, most of the difference between "closed" and "maximum flow" is in the first two turns from closed. So adjusting from fully open to half open to three quarters make next to no difference.
I'd suggest turning of the heating for an hour, so all the radiators cool down. While this is happening, remove the retaining screw from all the lockshield caps. Under each, you will find a spindle, usually flattened at the end. BTW, old valves sometimes leak round the spindle. If that happens, quickly screw it all the way open, or all the way closed, and it will probably stop. You must however replace it soon.
Using your smallest adjustable spanner, adjusted to be a tight fit on the flats of the lockshield spindle, screw it down to fully closed. Count the number of turns from its previous position. If it takes a lot of turns, you can assume it was wrongly adjusted. Do that to all the radiators. The TRV or knob at the other end should be set to fully on - that one is an on/off, not an adjuster.
Now turn the heating back on, with the room stat set high. Go to the nearest radiator and open the lockshield by one turn. Feel the pipes at each end. Within a minute one of them will have started warming up. If not, open the lockshield another half turn. Once you have opened it enough to get some flow, do not adjust it again by more than a quarter turn in either direction every ten minutes.
Get a notebook and keep track of how much you have opened each lockshield.
Adjust the lockshields on all the other rads by the same amount, just enough to get one of the pipes warm. Within ten minutes you should find that the tops of all the radiators are hot.
Now feel the pipes again. The pipe at the incoming end of the radiator should be "too hot to hold". The pipe at the other end (return) should be less hot. You cannot be sure in advance which will be the flow and which the return.
If any of the return pipes are also "too hot to hold" then that radiator is taking too much flow. Screw down the lockshield by a quarter turn and wait ten minutes. Go round and feel all the other radiators. The heat should be spreading downwards from the top. If any of them are warming unusually slowly, open its lockshield by another quarter turn and feel it again after another ten minutes.
By now, all the return pipes should be hot. The should not be as hot as the Flow pipes. They should be "too hot to hold for long". If you happen to have pipe thermometers or an infra-red detecting digital thermometer, you would like them to be about 20C cooler than the flow pipes, but if not, your hand will do.
After half an hour the temperatures will have stabilised. Feel all the radiators and all the pipes. You can now adjust where necessary, but by no more than an eighth of a turn in half an hour.
The room where the thermostat on the wall is fitted, adjust that radiator to warm up slower than all the others. If it has a TRV (it shouldn't) turn it up to max. This is because, once that room reaches target temperature, the heating will go off. If any of the other rooms are slower to heat, they will stay cold.
Now fit the covers back on the lockshields and keep your adjustment notes with the boiler instructions. If anybody fiddles with the lockshields you will have to do it again.