Changing the category a book is in

Does anyone have any success in getting a category that a book is in changed on .co.uk?
I have completed the form on Author Central only to be told that only the publisher can request changes.
I tell Amazon that I am also the publisher.
Amazon then say that the book is “locked from us” and no changes can be made.
Then, “Since you are this book’s publisher, please submit your request using the same website where your book was uploaded on Amazon.” However the book is loaded automatically onto Amazon’s pages via Nielsen who allocate the ISBN. They don’t have Amazon’s book categories.
I used to be able to do this with Amazon. What’s changed?
My book remains in the wrong category!