A year ago I received the following response from Amazon when I inquired about review appearance on US vs. other Amazon sites. I wonder if they have done anything about the inconsistent cross-linking of reviews in the past year.
Currently, there is no option in Author Central to share/ link these reviews on Amazon.com, as our international websites operate independently. Our Customer Reviews feature doesn't link reviews across Amazon.com and the International Amazon sites.
We're always looking for ways to make our service more useful to authors. I've passed your message to the Author Central development team for consideration. We rely on author input as we make future plans for Author Central.
However, if two or fewer reviews exist for an item on Amazon.co.uk, additional reviews for the same item may be shared from Amazon.com. Reviews that are shared from Amazon.com are marked as "Most Helpful Customer Reviews on Amazon U.S." Your Amazon.com reviews will appear on all our International websites.
But reviews from International websites do not appear on Amazon.com.
[my bold, italics]
I'm sorry for this disappointment. We’re working to make all Author Central features available to all sites.
As soon as more than two Amazon.co.uk reviews have been written, any reviews shared from Amazon.com will be automatically removed.