People just don't want to buy their cd's anymore. Hell most people probably don't like to buy singles of artists they're not big fans of... I've come to believe that single sales do not reflect what the general populus thinks about the song.
Sexy Boy is a infectiously addicting song, but I don't think it will get casual listeners to get up and spend their money on it. They could make one of the greatest H!P songs ever and I doubt that sales would make a significant rise in sales. The only thing I believe can do that is if they add extra goodies that people actually would want.
Some people might think that "illegal" mp3 dling is one of the causes to the drop in sales. As someone who is a fan and someone who dls music, this is true. But if you think about it, people are not going to dl a song if they don't like it. I think it's a good thing because it gets people into the artist relatively cheap and then they spend their money on the things that really matter; like concerts, photobooks, dvds, and other goods. These are the things that get them money. Just look at the crowd in the recent sports fest. The stands were filled with people.
CD sales have been low for a while now, but they are still concidered to be the top female idol group in japan. You can't blame UFA for low cd sales, blame the market that they're selling in.