Check the email suppression list in Sparkpost.
After logging in click on "Recipients" and then "Supressions". Then search for email and make sure they are not on it. If found, view the details to get more information and then "Delete"
Video
https://getschoolnow.slack.com/files/U12GYF275/F04UHQR9327/2023-03-17_10-50-56.mp4