This can happen if the parents do not have the Parental Access and List as Parent fields marked.

1. Go to the Candidate's Contact Card
2. Locate the Parents
3. Ensure that they have both List as Parent and Parental Access marked.

If the parent does have both of these fields but the Responsible Signer field is still not marked follow the steps below to ensure that the relationship settings are correct.

1. Go to Core > Users > User Profile Settings > Relationship Types
2. Ensure that the Parent - Child Relationship has the Responsible Signer box Checked. 
3. If the box is not checked, select Edit and check off this field.
User-added image

Note: This will update this information for users moving forward but will not retroactively update this information. A data request can be done to correct existing records that do not have this field selected.