I just repsonded on vbulletin.com, but this is probably a better place to discuss the kind of thing I was talking about. What I posted over there is:

The only thing I can think of offhand is that when the forum is in debug mode, you can display the queries that were done when building a page, and if you click "new posts" you can then see the query of the thread table and what the cutoff timestamp is. I'm not sure if that will help figure it out or not. I suppose if you're not getting any results then it must be that the cutoff time is in the future, or there's an error of some sort.

OK, I looked at the code a little and looking at the query might help, because the other possibility is that some forums are being excluded for some reason.