Happened to me when I saw a book on the US site, I wasn’t able to see all reviews, but there was a button to sort-of “apply” to get approved for seeing all of them.
It took five days, then I got this email.
Interestingly, they’re not doing this on the European site, you can still see all reviews there.
“To improve the user experience we are hiding things that were one of the primary reasons we became a monopoly.”