Filter does not display sub-category products

Hi,

This filter will remove all the products that aren't in the category selected by the user on the products listing he is seeing before using the filter.
So make sure that you've configured a menu item to display all the products so that it can filter them out.
Also, a filter doesn't work well if your products are in level 2 sub categories as they aren't in the selected category as the filter only allow you to select the categories of the first level.
In such case, instead of using a filter, I would rather recommend to use a HikaShop content module configured to display a listing of the categories. It can even support 2 levels of categories like you can see on the left sidebar of our demo website:
demo.hikashop.com/index.php/en/