How come?

How come some mothers don't form an immediate bond with their newborn???
​I was watching a movie called A Mother and A Child. And this women who couldn't have a child of her own, finally adopted a baby girl. She was struggling to take care of her and after like a week or two in the movie she then she turned to her mother and said, "I don't love her."  
​Why is this? What causes this? And if it's normal, how long does it usually last?