cannot open downloaded library books on Kobo Aura H2O

Hello,

We're having some issues opening library's ebooks on the Kobo. The books are syncing up just fine, but when we go into the device's bookshelf, the error reads "your device is not authorized to read this book" (or something to that effect), even though the we have authorized it through ADE.  Also, in the bookshelf folder the downloaded book is displayed as a black thumbnail. Kobo's software version is 4.5.9587 and we're using a Adobe Digital Editions 4.5. Any help is greatly appreciated!!