I can think of two possibilities. One is that maybe you have the threads arranged oldest first? Just above the thread listings is a blue header with “Title” on the left and on the right “Last message” with an arrow next to it. The arrow should be pointing down. If it’s pointing up, it’s showing the oldest threads at the top. Click “Last message” to switch to newest first.
Either that, or you’re looking at pinned threads. If it’s neither of those, I’m out of ideas.