With the wi-fi Nook, we have experienced some difficulty accessing downloaded books on occasion. Although the eBbooks have been purchased, are in the account, listed in the device, and have been read by a student in the past, the books sporadically disappear and are unavailable to read.
When this occurs, students will try to access a book, but rather than opening the book, it will go to the sample page where you can read a synopsis of the book or download the book. If you try to download the book, the message that appears will ask for an updated credit card (if you don't have one on file), and then prompt you to call customer service.
We had this issue again today, so I contacted a Nook representative. I was told that the time zone needed to be checked and daylight savings needs to be turned off. Although the time zone was correct in the past, it had switched to Eastern European, so the time was not correct. Did the time zone switch by itself or did a student change it? Both are possible but after speaking with other Nook owners, they have encountered the same problem, so it appears to be a Nook issue that hopefully they can fix. Until then . . . if you can't access your ebooks, check your time zone and daylight savings settings on your Nook.