The Mobicool windows with a white frame on the inside that have a retractable blind and screen combination seem to give quite a bit of trouble. From what I can read on the internet most complain about the screen not retracting.Before you pull off the inside frame though try a silicon spray on the slides to see if that fixes it before you pull off the frame.
I have pulled off the internal frame and found that there are a couple of things that can cause trouble. The insect screen can come loose off the spring tensioned aluminium roller causing it to let go. Another source of trouble is the brush seal that is in contact with the screen as it rolls up can be a bit tight. One solution is to remove the brush seal. ( it just pulls off the frame) and discard it. If the screen has come off the roller, (its glued on with contact adhesive which can dry up and it lets go), re-glue it with 5 minute araldite and leave it set overnight. The spring tensioned roller can also loose it's spring tension. There are two brackets that hold on the roller. Loosen off the one that holds the flat blade of the internal spindle (on the right hand side when the roller is close to you) and wind up the spindle to increase the spring tension and slip it back into the slot. A bit tricky but it can be done. I have not been able to locate any spares parts except you can buy the whole frame with the blind and screen as an assembly, but quite expensive. If you cannot fix it you have lost nothing!