Unable to add multiple Node Types in Hierarchy Set

Hello all,

I am very new to EDMCS and explored very little. In this journey, I came to know about hierarchy sets where we can have multiple node types. So, I have registered an EPBCS application along with 2 dimensions (Department and Employee) as shown below.

As known, while registering the application, EDMCS will automatically create corresponding Node Types and Hierarchy Sets.

I have created a custom Hierarchy Set namely "Sample Hierarchy Set". When I am trying to add multiple node types, EDMCS application is not showing other Node Types when "Manage" button is clicked.

Just curious to know why other Node Types are not being shown and the grey area that I am missing here.