This actually happened to me after connecting to wi-fi but there is a workaround that I found:
Convert your book to .azw3 in Calibre
Instead of sending it to the device in Calibre, locate the azw3 file (Right click -> Open book folder).
Copy the file to your Kindle, but not to the "documents" folder (where Calibre usually puts it) but rather into Downloads->Items
This folder is where books go when you buy them from Amazon or receive them after using the Send to Kindle feature. I have only tried this with azw3 so far but it might also work with .mobi format.
Convert your book to .azw3 in Calibre
Instead of sending it to the device in Calibre, locate the azw3 file (Right click -> Open book folder).
Copy the file to your Kindle, but not to the "documents" folder (where Calibre usually puts it) but rather into Downloads->Items
This folder is where books go when you buy them from Amazon or receive them after using the Send to Kindle feature. I have only tried this with azw3 so far but it might also work with .mobi format.