I have had a lot of trouble with WhisperSync and spent countless hours on the phone with Amazon and Audible regarding more than one book. At this point I have pretty much given up-if it works, great; if not then I just manually find my place.
I know it is not my devices, bc it responds in the same way at the same time on all of them (Fire, ipod, Kindle keyboard and K4.) Sometimes it works, other times it does not, or only works one way, usually audible to book. The odd thing is, sometimes it will work on a particular book, then later, same book, same devices, it will not work. When it doesn't work, it won't even work when book and audible are both on the same Fire. I tested one book, and it worked during parts of the book, then in other parts it would not work-same parts every time.
Sometimes it snycs from, let's say Chap 6 in Audible to Chap 6 in the book, BUT the chapters don't line up; Audible usually makes their "chapters" longer (an irritant.) A contact person at Audible acted surprised and said the chapters should and usually do (!) match up and speculated that was the problem.
Great idea, but I have wasted too much time being frustrated with the inconsistency. I think Amazon and Audible recognize problems, but don't acknowledge them-they always act surprised, make me do the same resets, tests over and over, and ultimately offer a refund and/or say they need to investigate further and will get back to me-they don't.