Showing number of topics in categories and subcategories

Hi!

I have set up a forum with three levels:

1. Main categories
2. Subcategories
3. Sub-subcategories

I can't figure out, how to show the number of topics in the subcategories. If there are 2 topics and 1 answer in a sub-subcategory (=level 3), it still shows "0" topics and "0" answers in the corresponding subcategory (=level 2).

Why does the subcategory (=level 2) not count the topics of its sub-subcategories (=level 3)?

Thanks for any help!

ChrisV