Associating products with their respective category requires checking the Parent Category box as well as the Child Subcategory box if you want the product to show up from both the Parent Category link and the subcategory link. It is possible to click on the Parent category link and be informed that “There are no products in this category” and yet below the message will appear a list of products. This list of products will have their association subcategory box checked but not the parent box checked.

So to get rid of the message: “There are no products in this category” make sure the products have their parent category box checked as well as the subcategory box.