Is it the Touch Screen flex cable that is on the way preventing to proper assembly? [br]
If yes, they go “hidden” behind the LCD and a little part goes hidden “inside” the chassis. [br]
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Is tricky and very hard to explain in words how to do it. I would recommend to look up for some videos “iPad Mini 3 Screen Replacement” and attention to details, this is the most important one.
Good luck!