I just figured out how to fix mine. You have to roll it up once so it pulls tight before to clip it back in.

If you pull it out all the way, with the rough edge facing down, you need to flip it up once so the rough edge faces up on the inside, so its tight when you clip it back up into the metal brackets. If you dont roll it once it will still be saggy.

Hope thats understandable, if not send me a PM.