I think one of the reason Ayu's popularity (and I know someone is gonna have something to say about this), which was something Foxxy_Fuyumi brought to light in another thread, is for the simple fact that Ayu releases TOO much TOO often...as opposed to Utada who releases things every now and then. Maybe it's just becoming too much for the Japanese public and they're getting tired of her material. Ayu does release singles/albums at a very rapid pace, even if it's not quite as often as she used to. It's a little puzzling, but I really think it has to do with Ayu's excessive releases, which just to let you know does not bother me in the least
I'm just saying that Ayu's crazy music + rapid releases might be getting a little old to the Japanese public, but talk about lack of loyalty to your favorite artist :/
But there is another theory I have...maybe it's Ayu's new recent style of music? Lately, her music has been more rock-involved and has taken on darker sounds to it, which if you look at her past sales on A BEST 2 -BLACK- (which is said to feature more dark songs than A BEST 2 -WHITE- that features the lighter and happy songs) there is a slight difference in the sales.
A BEST 2 -BLACK-
http://wiki.theppn.org/A_Best_2_-Black-
*Notice it actually reached #2.
A BEST 2 -WHITE-
http://wiki.theppn.org/A_Best_2_-White-
*Notice this reached #1.
Also, glitter/fated sold better than talkin' 2 myself/decision...
So who knows? It could be a variety of things causing Ayu's sales to drop. She certainly has an interesting marketing strategy.