Missing or grayed out PO fields

Hello,

I created a copy of the NB PO document type (in MM -> Purchasing -> Purchase Order -> Define Document Types) that is linked to most of our PR document types.

Many required PO document (ME21n) fields are missing (net price) or grayed out (e.g. short text, matl group, delivery/invoice tab fields). I've reviewed the Doc. screen layout (Define Screen Layout at Document Level) but I'm unable to link the new document type to an selection key (same as NB).

How do I link a PO field select key to a specific document type?

Nicholas