I told her she's beautiful, but she doesn't believe me. What is wrong with her?

There is a girl I like and I would like to date her.

Once I told her she's beautiful and very cute, but then she replies "no I'm not beautiful, I'm fat! " (well she's a little bit chubby but nothing much, I like girls with a little bit of meat).

every time I make a compliment about her she don't accept it, why? Is it because she's not interested by me?
I told her she's beautiful, but she doesn't believe me. What is wrong with her?
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