I really don't understand what makes my problem
I meant back-to-back flashing method. I tried it on THREE N9s and it failed!
On other N9, it works like a charm. Can firmware version be a problem? On the N9s it doesn't work, there are 001 and 009 firmwares, on the one that work there is 342 firmware...
But I don't want to upgrade. I just want to know why it doesn't work on most of N9, while it should!
And a fix for that. Haven't find anything yet.