My AU I Forte had exactly the same issue - where the rear drivers side door would not lock. I went through the whole "check the worm drive gear and electric motor" process, however this was not the issue.
The problem:
I found that the connection between the locking mechanism and the inside handle is the problem. When the inside handle is used to open the door the handle will return to its original position (so everything looks ok) but the locking mechanism will not. When the locking mechanism is not back in its original position not only will the child lock switch not be able to move but the door will not be able to lock. The easiest way to test for this is to see whether the child lock can be moved up and down.
The solution:
Dunno... A stronger / thicker gauge wire between the inside handle and the locking mechanism? This will allow the handle to push the locking mechanism back to its original position, hence allowing the door to lock.
The dodgy solution:
I found the easiest way to solve this was to disconnect the inside handle from the locking mechanism and to tape the child lock switch into the down position. This way the locking mechanism will always return to its original position and hence the door can lock.
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