You are aware that it needs to be the .mobi file of the book you add to your Kindle? Though I guess it doesn't matter as Calibre will automatically convert an .epub to .mobi when you send it to your Kindle.
It could be a problem with the Kindle being stuck on indexing a book.
To check if this is the problem, from the home page on your Kindle, do a search for a nonsense word (like "xxyyts"), and on the search results page if it says something like "xx items not indexed", click on the number and it will list the book titles not yet indexed. You then need to delete these books from your Kindle (hopefully it will just be one book).
If it's just one book it's worth doing a .mobi to .mobi conversion of it in Calibre before then re-adding it to your Kindle, and hopefully this time round it will appear ok on your Kindle.
If it doesn't say any books are still to be indexed, then that's not the issue, you could just try rebooting it to see if that makes any difference (press and hold power button for 20 secs to reboot).
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