What sort of a Nook? Dedicated eReader, or tablet.
If dedicated eReader; it may be a bug in ADE2.0. Try the older but more reliable 1.7.2 and see if it helps.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
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
If a tablet, what sort of tablet and what software app are you running on it?
Some tablets/apps do not interact directly with ADE. For example the excellent Bluefire app.
You can still share books by using a file manager to copy between PC/Mac; and making sure ADE on the computer and the tablet app are registered to the same AdobeID.