A problem with the Kindle platform is that you might ruin the progress sync by opening a book on a different device before it is synchronised. The platform sees two bookmarks and chooses the latter, which is wrong. I've done this between my Kindle and my phone, because my phone didn't have a data connection right away to load the latest bookmark online.
Mokalus of Borg
PS - An unreliable network connection should have been designed for.
PPS - Because our wireless networks are significantly unreliable.