How do I select a Part subtype when creating a New Downstream Part in MPMLink?

Version: Windchill 13.0

Use Case: In BOM Transformer, I want to create a New Downstream Part, using a Part Subtype that has already been created in Type and Attribute Management. However, the part drop down list isn't populating.

Description:

I have already created a new Part Subtype as a Site Admin and I want to use it as a new Downstream Part in the BOM Transformer. However, the dropdown list is only showing the very top level PART as an option. Is there a setting or something I am missing to allow all of the subtypes to be shown?