I also have problems getting my tablet to update. It's the 2012 N7 16gb. Stock/rooted.
I've gotten the update, and downloaded it, but when it reboots, it takes me to TWRP. Rebooting from there does nothing and the tablet stays on 4.2.2. My Gnex (same setup) updated with no problems.
My guess is an app, but I don't know which one or where to start looking. It's a family tablet so there are numerous games/apps on here (all from the Play store)
Any advice before I resort to wiping and restoring from factory?