I THINK I have the same problem.
I downloaded ADE and authorized my Mac computer, over a year ago when I had an IREX ereader. Everything worked. Now I have a Nook and I wanted to read a book from EPUB. I opened the book file with ADE and it appeared immediately in ADE library. On a different page I see my Nook. But I can't get book and Nook on same page; neither can I drag the book. I can open and read it on the screen but the second I leave that page to go to the bookshelf list where I see my Nook, I lose the book totally from ADE.
This seems to be to be behaving as though my computer has not been authorized, even though it tells me I'm authorized, gives my correct address, and offers no opportunity to make changes. So I dumped this ADE in the trash and downloaded a new version. Darn if it didn't turn out to be exactlly the same, just as I'd left it, and behaving just the same. So what do I do now?
Add the book to your Library in ADE.
I've done that several times over; it escapes everytime go to the bookshelves.
1. I'm sure you must have tried signing in to Adobe and changing email address in your profile. Don't know if they retrofit.
2. I can't account for what just happened. Last night I took screen shots of all the various manifestations of my problem, including messages that the book both could and could NOT be copied. For the umpteenth time I reopened the original booksale into ADE and tried to drag to Nook--got the usual red X's. This morning the icon miraculously allowed itself to be dragged into the nook. Green all the way--nothing had been changed. Elves in the night? Then I opened Nook--no book! But there it was in Documents. No, it wasn't there all this time. I knew to look there because of other threads. I also remember someone else had a problem that sorted itself out after leaving the Nook attached to computer for a long time.
So thanks for all the suggestions. What I take away from all this is that the Nook works well with B&N but using ADE for EPUB books is clumsy and tedious. Or possibly it is just slow. If I had wanted only books in English I could have gotten the Kindle, but with Nook I expected to gain access to Dutch books. It had worked before, but this time, perhaps with this seller, I spent hours spread over 3 days to sort it out and I still don't know how to avoid this the next time.