iPod Touch 4th Gen No Power after screen replacement? (tricky one)

This problem shows up everywhere but NO solutions! does anyone know?

i currently bought a lot of ipods (18 to be exact) all 4th gen 8 gig.

after replaceing screens on a few with no problems i had one that was unresponsive after screen. then another and another. now i have 6 out of 18 ipods at are unresponsive AFTER i replace the screen.

HERES THE CATCH.

before i install new screen i check the LCD to make sure they work and they all DO. the screens are just fine. no flex cables are ripped, i examined entire board under microscope and everything is clean. and i mean EVERYTHING.

reading online this problem has been occuring so a long time and no one seems to have a solid answer.

anyone help? advice? explaination? somthing?