Using Joomla 3.8.2, PHP 7.0.98, Kunena 5.0.12
Background Info: Using Joomla backend / Components / Kunena Forum / Categories, I could not get my sections and categories to reorder into the desired sequence.
This is what worked for me.
Problem #1 -- When clicking on the "Ordering - Select to sort by this column" icon, the icon would blink and become inactive before I could click on it.
Solution #1 -- For me, I just had to move my mouse pointer a little down and to the right of the Order icon, until the label displayed without blinking out, and then I could click there, instead of on the icon, and the icon would activate so that I could reorder my sections and categories.
Problem #2 -- I would reorder my Sections, but then on the front end of Kunena Index, the sections would appear in a completely different order. Frequently, when I moved a section around in the middle of the back end index, it would appear on the front end near the top or beginning of the index. Repeat until totally frustrated.
Solution #2.1 -- When you move a section, experiment with placing/releasing it a little lower (or higher) where it goes between two other sections. Pretty soon, you will figure out just where to place/release the section being moved.
Solution #2.2 -- Clear your Joomla system cache before and after you move a Kunena section.
Solution #2.3 -- Clear your browser cache before and after you move a Kunena section.
Solution #2.4 -- Navigate away from the Joomla backend Kunena Categories page, before and after you move a Kunena section.
Solution #2.5 -- You may even need to log out and log in before or after you move a particularly stubborn Kunena section.
