how to fix my cassette double roller blind?

Just done mine.
If it has blind as well as net like mine you have to take the unit apart. Take out all screws across top of roller case. and the two each side. the side channels clip in to the roller case at top so just pull out leaving you with the roller case and of course the net and blind. look at the rollers and remove the end cap on the end that shows the black ratchets. place screwdriver into the ratchet slot and turn until you feel pressure keep turning to re-tension the spring gently release down pressure on screwdriver so ratchet end sits back in its retaining slot. if it tension is lost again then it may be you'll have to change the roller. They are buggers to do you have to count to ten frequently or it'll end up across the yard.
all the best

nobby
April 2008