12. They avoid people and social activities.
This is a sign that your foster child has withdrawn. Here is the problem. You could easily miss it. How? She may be very friendly with you, with other family members, and especially with adults. You say, “But everyone likes her.” The problem is “everyone” includes few if any children.
If she has really withdrawn from everyone, she needs immediate professional help. Something drastic is happening. If the withdrawal is from other children and activities outside your home, it is serious but not so much of an emergency. She still has you and your family.
Try two things. Talk with her about what is going on. She may tell you what is happening and what she thinks and feels. This will give clues about how to help. Also, back off a little from being her best friend. You are the only mother (or father) she has right now. That is your job. If you are her main friend, it keeps her from needing to find other friends.
You can help her spend more time around young people her age. You may need to push and insist a little. Stay close but give her room to be with other children. Also, she likely will benefit from being in a group at one of your community agencies.