I don't think it's anything to do with them personally, but so far nobody just gives a shit. We already have The Saturdays, Sugababes and to a lesser extent Mini Viva and The Veronicas on the market. GCH don't stand out at all. They're just a bland pop group with some safe catchy songs. If people see it on the music channels and like it they'll buy it, but next time they hear that they have a song out they won't go "oh i'll check out GCH's new song". They'll only buy it if they hear it and like it.
The Sats overcame that problem with 'Up'.