rewards bookshelf credit not working properly

Today marked the third straight time I have purchased an audiobook on the bookshelf list and had the code not work.  I have no clue as to why this is happening.  The first two times I got on a chat with Amazon and they refunded the $10 I shouldn't have been charged (they used a gift card the first time and issued an Audible credit the second time).  Today I took screenshots that showed the book was eligible for the credit and should cost me $10 less than what it did.  I'll get back on the chat with Amazon again, spend 15-20 minutes walking them through it, and likely have it resolved favorably.  Does anybody have any ideas as to why this is going on and how to avoid it?  I am 100% sure that all three times the audiobook was eligible for the promotion.