Is it possible to have the filters on sub categories aswell as the parent categories?
You can go to Catalog -> Manage Categories, on the left you will see your category tree with parent categories and subcategories and on the right in the tab "Category Products" you will see goods assigned to selected category or subcategory.
Hint - if you don't see goods and you are sure that this category should have them, then press thee 'Reset Filter' button.
Hope this helps.
Sorry i should have mentioned that its on the front end that i would like my customers to be able to filter products when viewing on a sub category