Agree with the other answers here. The ribbon cable is exposed between the volume-lock button and the power button and is very susceptible to being torn. Especially if you are not using caution when lifting the digitizer off originally, I've done it plenty of times.
I even had the ribbon tear in the same spot just from touching exposed adhesive. As soon as I lifted the digitizer to reposition it, the adhesive grabbed the exposed ribbon and ripped it right off. I circled where to look below.
A few questions to see if I can help further:
1. Have you adjusted/replaced/done anything to that volume/power cable at all?
2. Does the volume-lock and/or volume up/down buttons still work?
Another issue I have run into is if you tighten the screws down too tight on this cable (be it the volume section or the power section) the buttons wont work because they dont have wiggle room. You might want to get below the rear-camera and try loosening the two screws holding the power button just a tad. One turn out, one turn in; just try to play with it a bit to see if it works.
The benefit of fixing an ipad is that you don't have to have the digitizer or LCD set in to the housing to test these things over and over. The only thing you cant use during the testing phase is the home button (and there are ways around that too).