Hi Henri,
You have to enable the option Settings>Layout>Forums View>Show Categories with Empty Posts because the behavior is if your parent category has no posts, the posts from the sub category will not show on the Forums page.
I have helped you enable that option so you will see the posts from subcategories.

Also, do note that the Forums page only show the names of 1 level of subcategories. If you have subcategories under subcategories, their posts will show, but their name would not show in the Forums page(http://take.ms/K3dLi).

Perfect, thanks Raymond !

Hey Henri,

You are most welcome. Glad to hear that your issue has been resolved now.

As a gentle reminder, kindly start a new thread if you have any other issue in the future so it will be easier for us to manage your inquiry. I will lock and mark this thread as resolved.