I don't live in Japan, so I did all the usual hoop jumping to get it up and running last year, during last year, I have bought 7 books and it worked. I have not bough a Japanese book in a while but decided to give start buying again, and to my surprise it didn't work. I will clarify below. Everything worked as usual, I bought the book from amazon.jp website. I go to deliver it to my kindle PC program/app and it never downloads so I can start reading. I even deleted the book from my purchase history and bough it again, along with another book, but nothing. I can't get it to download no matter what.
perhaps a region lock like they used with DVDs ? dunno ... tried the usual stuff like vpn or manual override of your position ?