I cannot get my devices (kindle paperwhite and kindle apps) to sync to last page read. I have looked online and the only recommendation is to make sure whispersync is turned on, which it is. It is very frustrating, and I have concerns that it won't work when I am reading a kindle book and doing audible together.
some paperwhites require you to turn it on on the device as well:
menu (three dots in upper right)
Whispersync for Books = Enabled
ETA: in addition to what Pirate said, close the book and sync (menu option or icon) your device/app to your account before syncing the second device/app to your account and then opening the book on that device/app