The way the forum determines "New Posts" is via browser cookies.
Whenever you load a forum page, your last visit time is updated in your browser cookie.
If you then load the "New Posts" page more than 15 minutes later, the forum will only show posts made within the last 15 minutes or so.
Thus, it is a good idea to click the "New Posts" at the beginning of a new forum browsing session if you do not want to miss out on new posts made since your last visit.
Note that there is also a "Today's Posts" shortcut in the Quick Links submenu which shows all posts made within the last 24 hours, regardless of your browser cookie data.