Can a Recently Married US Citizen File For Her Adult Step Daughter?

I am a U.S. citizen, and I recently married my boyfriend. I filed for his green card and I also filed for his daughter, my step-daughter. They denied her visa application. They said that she did not qualify for a green card through her father’s marriage. She was only 20 years old when we married. What do you think happened?
If you married her father after her 18th birthday, then she is not recognized as your step-daughter for immigration purposes. If she is still unmarried, her father as a permanent resident, may file for his unmarried daughter, or if he becomes a U.S. citizen he may file for her even if she is married.