Adjust underfloor heating - defective?

You are right. I can turn the glass, but it doesn't change anything. I can remove the gray part on top. I have turned the knob underneath it a little bit. Let's see if anything happens.

I slightly turned the knob underneath.

The whole thing is like a thermostat for a radiator. You have the pin in the middle. It is pressed in by the motor or can come out further. That determines the amount of flow. You can play with that.

ah great, I got it. I have the feeling that the floor in the bathroom is only cold and not ice cold anymore. let's wait until tonight / tomorrow to see what happens

Does the sight glass now show flow? Otherwise, turn it fully counterclockwise.

Only the middle sight glass indicates flow. However, it was present in all other rooms except the bathroom. I turned counterclockwise at the point where the screwdriver is pointing. The red parts at the bottom cannot be turned, only the white plastic around them. I also wonder: is the bathroom the hose on the right or the second from the left - since the hose is labeled with “5”.

Oh, I have now found the key for the red ones.
Are all of them turned fully "on" counterclockwise?
I adjusted a bit clockwise.