Discussion board "NEW" tags dont show

I wondering what has happened to this site over the past few days. I used to be that only "new" posts were labelled as new. Now every day when I log into the page, ALL the posts are marked as new. It's very frustrating to have to mark them, day after day, as read only to find the following time I log in that all posts are again labelled as NEW. What is the problem? Is it in something I'm doing or not doing? Thanks.