Backlight not working after screen replacement

Hi,

I'm a professional repairman and in this case I need help: I've replaced an iPhone 6 screen and everything was working well. One day later, my customer called me and said that the backlight is not working anymore. It worked for one day and now it doesn't. You can see everything on the phone's screen, the touch is working etc. but there is no light. The slide control in the settings has no influence on the light. I tried the same with a new and broken displays as well but there is still no light. That means that the display is not the problem, something is wrong with the iphone. Does someone know the problem? How can I FIX IT??