Sub-category topics not showing on index


Currently topics sent in sub-categories on my forum doesn’t appears on the index page of the forum (which is set to display a list of the categories and it’s latest topics). Almost a month ago it used to show, and then it suddenly stopped showing new topics. Is there a configuration that I might have changed and don’t know about it, or is it a bug?

A (kinda) more detailed example of the issue:
> Main category
>> Sub-category
A new topic on Main category shows up on the index, but a new topic on sub-category is only shown when viewing the main category or when viewing the latest topics page.

Alright, it was just a configuration issue. Fixed it now.
To be specific, it was the “Topic List Sort By” category setting.