Occasionally, my Kindle will simply refuse to pick up any new books for a while -- this happens even with official Amazon-bought content!
It's only ever happened once or twice, and I suspect some weird firewall issues may have had something to do with it, but... take it for what it's worth.
It would be interesting to know if any other emailed books, or purchased books (freebie test?) show up while these two are being stubborn.