I see it now, you are using the navpopup gadget see in your preferences. The gadget works on articles and category pages without subcategories just fine, so I will have to take a closer look at why. I moved the JS to an import type off of our server as server caching seemed to be causing the issue. Well, that and the use of categorytree extension combined with the navpop.
I did see there is a JS error being thrown so I will report it to the developer/maintainer of the navpop gadget.
Seems to be working ok now, but I don't regularly use it.
Tom