Are you sure your computer with ADE and your eReader are registered/authorized with the same AdobeID?
You cannot copy at all if the computer is 'authorized without ID'.
Whoops, you mentioned you hadn't had the problem before, sorry ...
Are other books still copying properly?
If not, ADE might have got its registration/activation confused and in a semi-authorized state.
Try completely removing any authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your Adobe ID.
If so, maybe the book as particularly restrictive DRM.
Check the info on the book (ctrl-I to ADE while the book is in focus)