2ds xl screen brightness

Hey, I just got into referbishing electronics and ran into a problem. I have 6* 2ds xls and have replaced the top screen on 3 of them. The problem is that the new screens seem dimmer than the old broken screens. I called nintendo and they said that there is sometimes varation in each screen so i though it was just me. But after testing a few games and watching reviews for the 2ds xl online i still think im doing something wrong. when i was about to fix the fourth one i tested the screen before installing it and it came to full brightness like the broken screens then seemed to dim itself to be like the other dim screen. Do you have any idea why the top of my systems screens all seem to be dimmer than the lower screens or the older top screens. Is it a software thing? i cant seem to get any info online and i dont have a proper working example to reference. If you could help i would really appriciate it, thank you.