[lostinroutine]

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.