Just ported all the information over and people are making posts, but none of them are showing in What's New. I've already rebuilt the Search cache and that took forever. I'm loath to rebuild another right now but if that would do it then I will.
When you complete the import it rebuilds the thread and forum info. You shouldn't have to do it again. But it doesn't take very long if you want to do it again.
I can't think of any other reasons why new threads wouldn't show up in that list unless they aren't new, meaning you already read the new posts in those threads. Here is a small template mod which adds a link to show all recent threads, as opposed to just new threads.