The front passenger door on my 2015 X Trail wouldn't open from the inside. This wasn't a gradual fault. It happened from one journey to the next. After some research I have found the problem and fixed in five minutes.
The same lock is used on the front and rear doors. The front doors therefore also include a child lock switch that is inaccessible. This inaccessible switch moves under car vibration over time. To solve the problem simply remove the three lock fixing screws with a T30 screwdriver. Move the lock slightly and you will see a white sliding switch which is the covered up child lock switch. Move it to the opposite position (confirm by looking at rear lock) and viola....
I first saw this solution on a Qashqai form and I discussed with a)Nissan UK b) Ancaster Nissan Bromley and c)Ancaster Nissan Eltham to confirm. They ALL denied that the same lock was used and all denied that this could possibly be an issue!! This is my typical experience of these Nissan main dealers.......ie they don't have a clue about the product they claim to be expert in.