Door insect screen?

I have a horizontal fly/insect screen on a caravan I just bought can anyone tell me how the roller tension is adjusted as it seem to be set to high. When the screen is pulled out and locked into the apology of retaining hooks (two identical hooks that lock together ) the slightest movement in the screen frame the screen retracts rapid. Nothing showing on top of the roller box so assume their is no adjustment.
No name on it as no doubt its a 'cheapie' and manufacturer too shy to add their name to it.