Multiple Categories – How To Select Which One You Use?

We have products which are in multiple categories, however it seems it isn’t possible to only show Recommended Products from a specific category. We want to be able to show only products from the specific category, underneath the Parent Category, but it seems to only be taking those from the Parent Category?

We have the settings like in the attached image (Rock is a category underneath the primary category ‘Guitar Books’ yet it only shows products from the primary category. I tried to exclude Guitar Books in order to force this to only use the Rock category underneath it, but when we do that it shows nothing at all?