If you are clicking on "New Posts" when you come back later in the day, it will show you only the threads that have new comments on them, and hide the threads that were shown to you earlier, whether or not you opened them.
When this happens to me, instead of "New Posts" then I click on "Quick Links" and "Today's Posts" to look at the threads that I missed or didn't have time to read earlier.
Or, if you are interested in a specific category, say it is about Movies, then you can page down and click on "Talk of Movies" when you first open up TOTV.
Hope this helps and addresses your question.