My experience with the NOOK Simple Touch is that the ereader will only display a book's cover as a thumbnail if the image file is actually titled 'cover' in the ebook. (Such as 'cover.jpg' or cover.jpeg', etc.)
A program like Sigil can open a DRM-free epub book (such as from Project Gutenberg) and allow you to change the name of the cover image file to 'cover' so the NOOK can find it. You could even find a better cover picture and use that instead.
Unfortunately, books you have purchased from B&N are almost certainly protected by DRM, which would prevent you from tinkering with them.
Also, the NST has a file called .docThumbs that saves those thumbnail images. Unfortunately the thumbnails aren't deleted when the book is, so if you read and delete a lot of content there might be a fair amount of memory being used up by dead thumbnail image files. You can delete them when the reader is plugged into the computer.
Now, this was all on the Simple Touch, so I can't say for certain that it will hold true for your newer device.
Best of luck.
Thanks! These would definitely have DRM, so that option is out. I'll see if I can find the .docThumbs file. Thanks!
gecn If you still haven't been able to get this resolved; I recommend signing out of your account on your tablet. It's a little less disruptive than a full on reset and should help.
To do so, launch the NOOK Settings app and tap "My Account". Once there tap on on your account email a few times rapidly, and you'll be presented with the sign out option.
After Signing out just sign back in with your BN account info and you're library should sync again.
Hope this helps