[Kindle] Book keeps disappearing

Hello everyone,

I apologize if this is a duplicate, but I couldn't find anything directly related. I recently have been trying to add a book from Calibre to my Kindle, and it will say on Calibre that it is on the device. When I disconnect, the book is there for a few seconds, and then disappears. It can't be searched, it is not being indexed... it's just gone.

Any ideas? Thanks in advance!