[Cinisajoy]

Are these books from Amazon?
If yes, then Deb and Issybird are correct.
Let me do a little more research on other books.
Back from my research.
If they are your side loaded books, Amazon does not acknowledge those highlights as their highlights.
Now I did find something on Amazon books. I clicked the menu in the book itself and went to book extras. There is an options feature there you might try.
Book extras is not available on the docs.