This stumped me earlier, but I figured it out. The newer version of UBB resets the cookie that tracks new posts since your last visit when you come back to the forum in a new browser session (closing IE and opening a new IE, then coming back) rather than simply on a page refresh. There's a link on the "my profile" page called
"Update New Post Indicators" that resets the cookie on command. I made that link my bookmark.