ADE No Permission to copy the book

I have Adobe Digital Editions and need to transfr books from my ADE library to a book reader (Nook).

When I try to transfer books from the ADE library the following message comes up - "CE-COPY -NOT-Allowed - No permission to copy the book"

The Reader is authorised for ADE and I have unauthorised and authorised again because of the problem.

I presume this is something to do with ADE but please can you help me to resolve the problem.