I am seeing a strange behaviour

I have Document type "PM - A" whose page template name is also same "PM - A". below this I have document and page template type "PM - B". Now If I am placing a webpart inside PM -b as shown in figure below, its not rendering. However If I am placing it at a level of Pm - A its rendering. I have limited visibility of this webpart by allowing it to render only for doctype - PM -B. this approach is working , however in future this will create problems for me. Any idea, how can I debug this problem. I have checked what all document types can reside under one another. Any pointers?

- PM - A
- PM - B
- webparts