Document Sequences Set-up,

We currently have 4 seperate entites in our Italain set of books and we have set-up the Document categories and sequences for each entity, as your probably aware the document sequences are driven by the set of books, so when users are entering Invoices for each entity the Document category name does't automatically populate and the user has to manually populate this. Can you anu help and let me know if I'm missing something in the set-up.