The reason for this may be that they don't have an Enrollment Row for next school year. To check go to:
  1. Core
  2. Users/Access
  3. Profile
  4. Search for and select the student
  5. Scroll down to Student Enrollment under System Information
You should be able to see if they have an enrollment row for the next year. If they don't, you can add it through:How do I add future enrollment rows to a students enrollment?