The underfloor heating on the first floor is no longer working

No, unfortunately it is not that simple. Open also means 0.25 turns. You cannot determine that visually, and turning it without rhyme or reason will not get you anywhere.

No, unfortunately it’s not that simple. Open also means 0.25 turns. You can’t determine that visually, and turning it without sense and understanding won’t get you anywhere.

small update I have now vented the floor 6 times, each morning and evening for 3 days and look, it’s running again, but not 100%, some rooms are cooler, some warmer, so quite uneven, if you close the red valves on the ground floor, it logically heats up faster on the floor above. Could it also be that the pump no longer exerts enough pressure? or that the valve inserts are sticking, see picture 1, I have noticed that some move loosely when you press the nipple in and some are tightly stuck, I’ve also sprayed a little oil on them to make them move. And I have noticed that the screws, see picture 2, that are on the supply line are also all loose, what significance do they actually have? Or is it also because we have different blue caps because some were broken, where can you actually get new ones, I asked already if you can buy them separately but apparently they only come with a new heating circuit distributor, those blue caps.

Before you start fiddling around wildly, you need to understand what they do. Normally, you only turn it once and set a flow rate that depends on the performance length, thus adjusting the heat. Don’t you have any room thermostats?

Do the metal parts below have a scale?
That should be the flow rate.

Before you start fiddling around wildly, you need to understand what they do. Normally, you only turn it once and set a flow rate that depends on the power length and thus controls the heat. Don’t you have room thermostats?

Nobody is fiddling around wildly, but we do have room thermostats; however, for financial reasons we did not have the electric actuators connected.

Do the metal parts at the bottom have a scale? That should be the flow rate.

No, they don't, you can see that from the first 3 pictures.