The most common reason for not seeing newest posts at the top, would be if you accidentally clicked one of the column headers, which changes how the forum sorts the posts.

To get back to newest post on top, click on the words "Last Post" at the top of the column, so that there is a small upward pointing triangle next to it (you may have to click it twice, once to select, and once to change the order) This will sort the board by newest first (downward triangle is oldest first).