For who is interested my problem has a happy ending. What I noticed when I looked up close is that the screw that holds the plate (close to one of the holes where the hook from the body gets in) was not screwed in all the way. As result the plate was actually bent right above the hole and the hook was not holding well. Due to weight and gravity the back was falling every time. Couple weeks back I changed the light seal on that back and I left that screw half way in. As soon as I screwed in that screw, the problem solved itself and the back is holding well.
We keep learning . Cheers,