AAA
I used to love their music (well, mainly their singles) but their past two albums have not been very impressive. And I agree with megumi, their new songs all sound exactly the same. x_x They used to have so much variety it was crazy.
Do As Infinity
They were one of the first bands I listened to when I got into j-pop a long time ago, but now I don't listen to them AT ALL.
KAT-TUN
I used to be a really big fan of them, and I still do love their music a lot, but it takes me longer to listen to their new releases and I don't follow them anymore.
Lead
Used to love them too, but I don't follow their new releases at all. I stopped at their fourth album.
MiChi
I loved her first singles, but I lost interest after her album which I didn't like much.
SHINee
I loved them when they first came out, but... I have totally lost interest in them. Some of their songs are still awesome though, like Replay.
Super Junior
I loved them when they debuted but I haaaaaate them so much now. XD
Nakashima Mika
I used to love her music but I never listen to her new releases anymore.
Amano Tsukiko
This is probably the biggest one for me. She used to be one of my favorite singers after I became a fan because of her works for Fatal Frame (my favorite game series ever). But even before she stopped releasing new music, I totally lost interest in her and I only listen to her like twice a year now.
Okuda Miwako
I loved her so much but she doesn't seem to have released anything new and I just don't listen to her anymore.
Shibasaki Kou
I love love love her first album to death but after that her music just became generic, bland j-pop to me.
DAIGO☆STARDUST
I loved him so much solo, but I don't care for BREAKERZ.
That was a lot more than I expected... x_x