My nooks worked with Digital Editions in that they showed up in the library and I could easily transfer my newly taken out library books to my Nook. Now it no longer shows up in the library and therefore I cannot transfer books to the Nook.

I own several Nooks.  They easily registered on Adobe Digital Editions and then I could transfer books to the Nooks.  Now when I plug them into the computer they no longer show up on Digital Editions.  Therefore I cannot transfer my library books to them.  How can I make them work?