You haven't mentioned at what age you adopted her. There will be biological bonding with her mother if she had spent her early stages with her mother. The child may find it difficult as she might be struggling to balance between the her biological mother and adopted parents.  She may not want to go as the lifestyle and comfort could be superior being with you all. Either way the child could be going through emotional disturbance within herself. Communicate openly with the child, probably the girl needs more love and compassion from you. Consult a Counsellor in your city who can help you as well as the child.