If the user is not marked "List as Parent", they will not show in the printed directory document.
You can check this by:
1. In Core, from the Users/Access dropdown, select Profile
2. Search for the student and click on their name
3. Scroll down to Relationships and click on Relationship Settings
4. Make sure the step-parent has List as Parent checked off
5. If not, click Edit in the top right
6. Check of List as Parent