We had the exact same problem with our Designer rear window shade (day/night). I think it is just too heavy, being the longest one in the unit. Solved the problem by putting a small screw in the frame, one on each side, back behind the side curtain where it doesn't show. Now, I just raise the blind until the plastic bar can go over the screw, and blind will not come down unless you want it to.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.