Missing Books on Voyage

I use Calibre to sideload ebooks to my Kindles. I have a voyage and a PW. I have loaded some books, all by the same author, onto my Voyage. When I disconnect my voyage, the books initially appear in the library, and then disappear. Loading the same books on to my PW is successful and they stay available.
I have accessed my voyage via windows explorer, and the books are present in the Voyage's memory. The voyage has the latest firmware. Since I am not having a problem with the PW I suspect that this is a voyage issue and not Calibre.
Any ideas or suggestions as to the cause and solution would be welcomed.