Hid all the posts that didn't answer your question and gave you alternative answers instead. The answer to your question is that when you use WalkingEvent(area), the event will try and create a path to that destination using an algorithm. However if that destination is not in a loaded region, it will fail. To overcome this, you can create a manual path:
To walk to a specific position, as others have previously suggested, set the mindistancethreshold to 0. So to reiterate, it only works "sometimes" because the area you are trying to reach is only "sometimes" loaded.