1. You should click the blue button at the bottom.

2. Maybe you have lots of forums and sub-forums which passes the limit of your hosting resources. Here is the answer: https://wpforo.com/community/how-to-and-troubleshooting-2/save-forums-order-and-hierarchy-problem/#post-11412

3. This could be a cache issue. Delete all caches in Dashboard > Forums > Dashboard admin page. If you have a cache plugin, exclude the forum page from your cache plugin. Here is the instruction: https://wpforo.com/community/faq/wpforo-and-cache-plugins/