Can't send books to Kindle.

[ErikaGC]

First off, I decided to remove the books from my device (kindle Keyboard) and fix them up a bit in Calibre and Sigil. The problem is this; After sending a book, I see a green check mark beside the name of the book, when I move to the next book, it removes the check mark from the previous book I sent and one is added to the current one I am sending. What is causing this to happen?
It is happening to books in series, like in the Black Dagger Brotherhood has 12 books or something like that, I can only add the first two before I can't send any more without removing the ones I put in the first place.

Please help me out. I have had this problem before, and somehow it is happening again. I have had all the books in my kindle before, so I know they can be placed in the device.