Mary V: When no one has added to a thread in the last 24 hours, it drops off the screen and is no longer visible, unless you search for it, or set your "age" filter to a limit of more than one day.

To bring a thread back to the top of the list, all you have to do is add any message to it. If someone wants a thread to reappear so someone else will see it, but has nothing else to say, they sometimes just post the single word "refresh."