Glo one book added and one deletes

I bought a Kobo Glo 8 weeks ago and after a few weeks when I loaded a new book from Calibre onto my Kobo another book was deleted! I used my 64 bit mian computer and my 32 bit laptop and the same thing happens. I called Kobo who were no help, and the took my Kobo back to the shop today and got a new one . After setup was complete I was loading the first 25 books from Calibre & the exact same thing happened. One book on one book off!
Is it me, Calibre or the Kobo????
any ideas????
many thanks
Intipuss