Took me a while, but I think I can. The topics that contain the category description (About the xyz category) are hidden from latest.
I think the easiest way to confirm that this is the case is to use the “edit description” link from the edit category interface.
That is the topic hidden from lastest. If that is the topic where the global pin doesn’t work, simply create a new topic in that category and put the content into that.
You are not the first one who struggles with that. Often replies to that topic aren’t noticed because the topic doesn’t appear there.