I always thought her voice got worse because
a. she does too many lives, she overloads her voice and doesn't soothe it properly.
b. she screams too much at her tours, this actually damages the voice a lot.
c. she smokes. I suspect ayu to be a smoker, since there was some paparazzi photo taken in night-club where ayu was partying with her staff. on the photo she was smoking, I remember she then even apologized for such a bad photo. I think it was 5-7 years ago.
d. the Miniere disease, which led her to deafness. of course, it's difficult to sing if you can't hear yourself.
these are my suppositions
