One possible reason is that the Nook is full.
You can check using a file manager such as Windows Explorer or Finder and looking at the properties of the Nook as a mounted drive,
ADE 2.0 does not work properly with some devices. Just a guess, but try ...
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172