This is because the Parents are not associated with a School Year. When the Student associated with the parent 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:
  1. Navigate to the module the form was created in (Academics, Enrollment Management, Extracurricular, or School Website)
  2. Content > School Forms 
  3. Select Settings
  4. Select the Recipients Tab
  5. Select User, and select + Add
  6. In the filter options use the Role filter for: Parent (leave all other filter options blank)
  7. Select the Search Icon (magnifying glass) 
  8. Mark the option for Select All
  9. Select: Add Currently Selected