You will probably find that one or more of the plastic tabs that retain the strings have broken off.

This video shows the situation and how to repair it by replacing the tab with a self tapping screw.
Repair Dometic Heki roof hatch

I did this myself and it is an acceptable repair, but not perfect. I improved it by grinding off the thread of the self tapper just under the screw head where the string will rub against. Also, the string has a bit of friction, not just at the screw but other points as well. Some silicone helps it slide a bit better.

Later model covers have spring loaded rollers to retract and roll up the flyscreen rather than concertina it. That may be a better design but I have also had to repair them.