In order for users to appear in a distribution group, they need to meet the criteria of the list(s) in the distribution group.
1. If parents of a candidate do not appear in a distribution group, edit the distribution group and see which lists the distribution contains.
2. Then go to Navigate to Lists > Manage Lists.
3. Click the Manage basic and advanced lists block under List Actions.
4. Locate, and edit the lists.
5. If the roles listed in the Filters tab's Global Filters are Candidate and/or Parent of Candidate role(s) and the candidate and their parent(s) are now in the Incoming Student and Parent of Incoming Student roles, then they no longer meet the criteria of the list.