Sorry you're having trouble. Are you using the OverDrive Media Console app for the Nook colors and tablets? Or are you using Adobe Digital Editions to download the books and then side-load them with a USB cable? If you're using Adobe Digital Editions then you may want to try de-authorizing and re-authorizing your computer as well as your device. If you have Adobe Digital Editions 2.0 or newer you should use these instructions and if you have an earlier version you should try to use these.
If neither of those work please give us a call @ 231-3750 to see what else we can do for you.