There are some interesting charts on this blog on Ai Otsuka's declining sales:
https://appears.wordpress.com/2014/0...oricon-charts/
It shows the decline of sales of once top-selling artists.
I don't think the decline in sales is anything Ayu can do something about. Japanese people seem to have a very short attention span when it comes to artists, as soon as they go on a few year hiatus (like Ai or Hitomi) the public doesn't seem to care anymore. Ai only sells a couple thousand copies nowadays and Hitomi's albums sell as few as 1000 copies. Even though I actually think Ai's albums are an extreme improvement over her best selling albums.
Once the novelty or cool factor of an artist wears out it's just hoping for the best. The same goes for Western artists. Female artists just don't sell as well as they once did.