This can happen if the Teacher was not added in the term the course started in.

To add the Teacher to the first term of a course:
  1. Navigate to Academics > Scheduling > Requests and Schedules > Classes
  2. Filter for the school year, school level, and term
  3. Expand the department and click Manage Sections for the course
  4. Add the teacher and assign a personnel type
  5. Save