It sometimes seems as though that happens when there is a lot of activity on the site. Any more recent post pushes a previous post "lower down" and often even a fairly recent thread may move onto Page/Screen 2 or 3 in a very short time. If someone responds to it, that thread then jumps up to the top of Page 1. If you haven't tried it yet, click on NEW POSTS in the narrow black strip at the top of the page. This will open a list of all posts you haven't read yet. Look carefully, as there may be several pages in that list.
To clear the list completely, click on QUICK LINKS in that same strip, and select MARK FORUMS READ. Once you have cleared the list, just start with NEW POSTS every time you check in. The unread posts/threads will show as bold. When you've read all you want, clear the list again. I have found this the best way to be sure I don't miss any posts I want to see before they slip into the archives and are harder for me to find.