She peaked as a singer and an artist too late in Girls Aloud's career.
To many casual listeners she wasn't much of a presence in Girls Aloud and unlike Cheryl, she didn't have being a footballer's wife and being on TV to kickstart her career and give her an identity. GA weren't like the Spice Girls where each member was branded from the get-go.
Yes, she had a lot more vocals, solos, and embraced her looks around the time of GA's 5th album era and on the Greatest Hits, but GA were on the wane by then, so I think it was too late for Nicola to develop much of a dedicated fanbase.
Another reason is that her lead single unfortunately was released at the same time as Beyonce's Run The World (Girls), which uses the same sample, and of course Beyonce is much higher profile, so people may have gone off her for 'copying Beyonce'.