That means two things are true.
- CC is able to fetch books from your Dropbox if they are there.
- Some books are not synced to Dropbox.
I suspect that your Dropbox is out of space, meaning that not all of your files are being synced. You can check this using the following steps.
- Using your favorite web browser (Internet Explorer, Chrome, Firefox, whatever), connect to https://www.dropbox.com/login. Log in to Dropbox
- You should now see something like the following.

- Click the smiley face in the upper right. You should now see something like this.

- Check to be sure that you have space available (XX of YY used) XX should be less than YY, ideally by at least a 2 or 3 megabytes.
You can also check the specific book that failed to download. The steps:
- While still connected to Dropbox using the web browser, navigate to your library. I think it is /Books.
- Look for the folder "Cerulean Sins". I might have the spelling slightly wrong. If you don't find that folder then the book is not synced to dropbox.
- If you do find that folder, navigate into it.
- Look for the folder "Hamilton, Laurell K. - Anita Blake 11 (XXXX)" where XXXX is a number that I don't know. Again, I might have the spelling wrong. If that folder is not there then the book has not synced. If it is there then nativate into it.
- You should now see the files "cover.jpg", "metadata.opf", and "Hamilton, Laurell K. - Anita Blake 11 - Cerulean Sins.mobi". If any of these files are missing then the book is not completely synced. If the .mobi is missing then the download will fail.