Top screen not working(backlight only)

I have bought one “new” nintendo 3ds (2015) that was working perfectly and one that was broken. I have swapped the motherboards and when I check it it was still not working. After trying few things I ended up with 2 broken consoles, now both of them have problem with top screen, every time when I am trying to put it back to one piece, the top screen would not work, it “works” but you can see that only backlight is working. I have bought new screen and check if it was it, didn’t help. Any ideas? Please