ADE not connecting to nook

I used ADE once to download Library books and it worked fine about two weeks ago.  When I downloaded more books to ADE a couple day ago, the ADE would not connect to Nook.  I could not get the Nook selection to show up.  Why did it work last time and not this time?  I really need help.  This has happened before.  I had to re intialize my Nook.  Then redownloaded ADE and it worked once.  Now the books stay on the PC.