Auto lock will always work if the fob is within 3 feet or so of a door when all doors are closed, then you walk away.
If the door is open and you start walking away as you swing the door shut, it won't work.
If a passenger or another person has a door open and you've walked away before they close it, it won't work.
If the rear hatch is open and you push the button to close the hatch and walk away before it closes, it won't work.
The hatch was the tricky one for me. If my hands are full I wait for it to close, then walk away listening for the beep. If my hands aren't full I start walking and use the remote to make sure it's locked.