My ebooks have stopped syncing to different devices

I have been using ADE for months, borrowing books from library, and downloading and reading them  on a Dell laptop and also reading them on my iPhone 6.  Suddenly they have stopped syncing.  I just authorized a different laptop and it isn't syncing either.  I have tried borrowing different books with no positive results.  All are authorized with the same Adobe ID and the automatic sync box is checked on all of them.

Any ideas?