There are various possible reasons.
There are various bugs that mean some do not work with ADE2.0.
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.
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
Tablets used as eReaders (including many Nooks) with an appropriate eReader app do not generally show up in ADE.
You need to copy books to the tablet using a file manager such as Windows Explorer or Mac Finder to copy to the mounted drive.
For Sony eReaders and maybe some others you need to install appropriate drivers before ADE will see the Sony.
For Sonys, installing Sony Reader will also install the appropriate driver ~ you don't actaully need to use Sony Reader itself at all after the install.
I don't know if that ever applies to Nooks.