Synced to wrong device maybe?
My guess is that you have done what I did at first. I did not change the selected Kindle device from my Mac to my iPad. My Macbook Pro was the default on the buying page. In fact now that I have Kindle on one of my iPhones (don't bother on a 3G - not really readable), when I buy a book, Kindle shows my iPhone as the default device. As someone posted above, you can go on the Amazon website to your account, then Kindle orders and finally manage your Kindle. You can then change the device for download of each book.
Wilson