I think you're getting the "Sorry, there are no new posts to view. You may search for posts updated during the previous 24 hours, here." which is saying that there have been no new posts since you've last checked the forum.
I have noticed that sometimes the What's New Tab is a little inconsistent with what it shows. Like if I check the forum on my phone it seems like everything will be marked as read and not show up when I check again later even if I didn't look at the thread in question, other times threads I've caught up on multiple times will persist in the tab.