I had tried several of the suggestions above and none worked. I could read my books on the Kindle Cloud, but when trying to sync with the Kindle app on my iPhone it would never sync - or vice versa. Rebooted phone, ensured whispersync was enabled on amazon site and on iphone, removed book from iphone and redownloaded, etc. I did one other task that made this start working again for me. I went to the Amazon site under Manage Content and Devices and under the Devices tab I made my iPhone the "Default Device" by clicking on the elipsis button next to my iphone. After doing this the sync started working right away as expected.