This is because the Parents are not associated with a School Year. When their Student is enrolled in school they are automatically a Parent for that School year until the Student withdraws or graduates.

To add Parents to a School Form, navigate to the form from the Product it was created in such as onMessage, onCampus etc
  1. Click Settings
  2. Select the Recipients Tab
  3. Select User, and click + Add
  4. Search for just Role of Parent and nothing else
  5. Click the Search Icon (magnifying glass) 
  6. Click Select All
  7. Click Add Currently Selected