You might check to see if the book has been updated (go to Amazon website and look under your Content and Devices). If so, make sure you have the updated version downloaded to your Kindle.
If that's not the problem, contact Audible. They will tell you to check if the book has been updated per above. Insist that you've already checked that.
When this has happened to me and I've already checked for an updated ebook, they do some sort of manual update on their end which fixes my problem. Hopefully the same will happen for you
(I've asked them what they do on their end and is there anything I can do on my end to fix it myself, and they say no, sometimes they need to do whatever they do on their end.)