Maybe you were still in roundtime from hiding or performing some other activity? You may already know this but... you can stalk while in RT but if the person you are stalking moves before you are out of RT you won't follow.
>Is there a problem with stalking folk in sanctuaries or if they are particularly young?
Not unless it happens regularly. There are a few situations which can potentially thwart stalking. There isn't enough information there to tell what could have blocked it.
