The pair have told the X Factor judge that they would like to concentrate on their respective solo careers before they commit to reuniting with the band.
"Tulisa thinks N-Dubz would be bigger than ever following her exposure on The X Factor but her old bandmates just aren't interested," a source told The Sun.
"Dappy and Fazer want to continue doing their own thing. They know Tulisa's solo career isn't going great and she's realised she's bigger as part of N-Dubz than on her own.
"Her record company are nervous after her last single and fear it could be over for her after one album. They blame the controversy that's dogged her since she went off on her own."
Tulisa will release her new single 'Sight Of You' in the UK on November 25, while her debut solo album The Female Boss follows on November 26.
Watch the music video for Tulisa's new single 'Sight Of You' below: