Default [Q] Help N7 with TWRP but no system

Hi all,

it's now a month since I downloaded the 4.4 update crashed my Nexus 7.
I have been trying several tool and solutions but nothing worked. I know there are many posts for this problems but I may need specific help.

Current status is I have a working TWRP recovery but every ROM I try to install I get "failed" without further explanation.
Before I was "Unable to mount system" but now it seems sorted out.

I also tried to flash the various components one by one using fastboot. Everything fine for boot, recovery and userdata, but when I try to flash system it gets stuck there. I waited for a few hours but nothing happened.

Any suggestion?