Dear Anna, I have had 2 similar issues in the past and the first one was a ribbon connecting cable that was possibly dirty. I disassembled the device, cleaned the ribbon cable with isopropyl-alcohol left it to dry, reassembled and boom! it worked.
The second time got in the same situation, tried my usual trick but it never worked. Sent back the display assembly to my supplier who confirmed that it was the actual display assembly at fault. Sent me a new one and it worked instantly.
Worst case scenario would be the display chip though unusual.
I would suggest that you begin with cleaning the display ribbon and see what happens. Good luck!