Dinky, I wiash I had a magic wand....

I am also thinking a new teacher has new eyes to see the situation...
Maybe the other teachers just let her go & do her own things or knew what she was like already & so just let her manage at her own way/pace... Not necessarily bad or good I guess!

I am not sure what you should do...

Perhaps discuss with the new teacher what can/should be done in order to get her motivated or to reach the std the rest odf the class is at? Perhaps you can expalin the situation youre in & that the other teachers had never mentioned what he has & so your wondering what to do now...

He may have some answers as to why the other teachers had let her go, or he may be able to suggest extar activities to assist her!

I'm really not sure..... Good Luck!!