Hi Fabienne, welcome to the forum. If you use the search tool at the upper right (magnifying glass icon), you will find a lot of posts if you type in - bees not using flow.
As @Webclan says, you first need to have a strong hive before you put the Flow super on. If that was true, then there are several other things you can do to encourage them.
The simplest is to save some burr comb when you are inspecting, and use your hive tool to push 1 to 2cm blobs of comb onto the faces of the Flow frames at about 10cm intervals. This makes the plastic smell of your hive by putting bee pheromones onto the plastic.
If you have melted wax from your hive, you can paint it onto the frames, again making it smell of the hive for the bees.
Some people have found that taking a frame from the brood box and moving it up to the Flow super can encourage the bees to move into the super. You have to provide an upper entrance for hatching drones to use if you decide to do that. Once the bees are in the super and actively using the plastic, you can take the frame out and replace it with the plastic frame.
I used the burr comb method. Within a day or two, bees were in the Flow super, sealing the gaps in the plastic frames. Within 2 weeks, they were using the middle 2 frames to store honey. Good luck!