Sandcastle documentation displays sub-namespaces in one library as different tree nodes

I am using Sandcastle to generate C # project library documentation. My project library has one top level namespace and some sub namespaces depending on the folder structure in that project.
For example, if one project library has two namespaces -
MyCompany.Product.FileHelper
MyCompany.Product.FileHelper.Enums

When I see the Sandcastle documentation for this project, the two namespaces appear as different tree nodes, but at the same level. I want any sub namespace to appear as a subtree.

FYI, the current Sandcastle project settings are -

Can anyone point out how I can display sub-namespaces within the same project library as a subtree menu item?

Thank!