I have the same problem. Books have been "delivered to" my kindle touch and my iphone via the amazon site.
Syncing used to work perfectly. I could read on my kindle and then later when I was out and about I could open up my iphone app and it would sync to where I left off.
Two things have happened lately. Amazon has sent us canadians to the .ca site to manage our kindles. And I have replaced my iphone.
I had a temporary solution by deleting a book from my kindle and then hard rebooting the kindle and re-delivering the book again. It synced to my phone for one read and then failed again.
The error I receive is this:
Unable to retrieve the furthest location read. If this is the first time you have opened this book, your device will update your location when you close the book or your device goes to sleep.