Have you seen the colour coding for Tokaikko Junjou? It's all Airi and The Rest sharing lines that aren't hers. (And even then, she sings the chorus with Maimi and Mai.)
True, Nakki actually got solo lines, and much more screen time, considering she hasn't really had any. But compared to oh, say Meguru which had them singing in sub groups, Tokaikko Junjou was Airi and C-ute. It's not as unfair with Berryz' Special Generation (basically Captain, Chinami, Maasa, and Maiha didn't sing), but the exclusion of Maimi from solos basically tipped the scales towards Airi.