This is likely because the registration badge was added to your program after this person enrolled. Try un-enrolling (click on the checkbox under "Enroll") and then re-enrolling (click on it again) to then earn the registration badge and any associated rewards.
To get to the screen to "unenroll" someone, you want to go to People > Find a Person. Then, search for the person and click on their name under "Reader." Click on the box under "enroll" to remove the check, and then re-enroll them by checking it again!
Note: This is not possible to do if someone has already completed the program.