I have learned that only pinned discussions can be reorganized by clicking and dragging. However, the unpinned discussion have the same vertical dots that to me say "click and drag to reposition". If the unpinned discussions can't be repositioned in this way, then they should not have an icon that leads the user to believe that they can.
An alternative is to use a pin icon/toggle that will pin or unpin a discussion. Pinned discussions would then acquire the 8-dot icon that would allow them to be ordered manually. The unpinned discussions would lose the 8-dot icon, indicating that they cannot be manually ordered.