You may need to refresh your browser to get rid of "new" flags when you’ve already viewed the new posts in a thread. I know if I click a thread, read the new posts, then use the "back" button to go back, it will still say "new" in that thread.
If I refresh, the "new" flag goes away, or if I return to the forum using the breadcrumb menu at the top, it goes away.
Jenn