One reason for this is the use of ADE2.0 in conjuction with a Sony eReader device.
With the eReader plugged in, ADE2.0 refuses even to display library books, let alone copy them to the eReader.
This may be an issue with other eReader devices as well; ADE2.0 is riddled with such bugs.
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