Robbi, if you search for ADE in the forums you'll find threads with some different things that worked for different users.
The ones I remember:
1) delete the book from nook.
2) redownload it from library.
3) Now move it over to your Nook.
actually, I'm not confident i remember that correctly. You'll have to search the forums.
This does not work, though some people have had short-term success with it. Thanks for trying!