The New Posts thread is the updated list of threads that have been updated by other people. If you then add a post onto a particular thread, your post would not show up under the New Posts.
I usually wait til I see that someone else has posted on the same thread after me, via the Summary Page (which DOES show your last post) or the New Page, so that I can find the thread again.
Hope that makes it clearer........I've now confused myself!!