It will only show when you are not filtering by a specific post. So if you see post_id= in the url but not topic_id = then it wont show. Which is what happens when you are using those jump to last post shortcuts. If you just click name of the thread from the topic list, then it will show. You'll notice that the topic_id shows in the URL and then the code that generates the Notification options box then is displayed. Bascially if that Topic_ID is not passed then that notification option is not displayed as it needs that value as Notifications are assigned by Topic ID not Post ID.