Kindle Fire - book not showing on device within Calibre.

Hi, I've never had a problem using Calibre until now, but when I tried to backup my recent purchases one of the books is not showing up in the main memory of the device so I am unable to back it up. I have tried removing the book from my kindle and downloading it again but that has made no difference.

Any suggestions?

Thanks in advance, and sorry if this is a question that has been asked many times before.