Why doesnt imessage work?

I have an iphone 4s with ios6 and im using imessage to text other people who also have iphones, but i have ONE friend that imessage never works. She also has a 4S and ios6 but whenever i text her its always with the green bubble eventhough imessage is turned on in both our phones and we both have internet connection. I know that imessage works with her because she talks to other people too using imessage. Why doesnt it work when its between us? What can we do?