Issue:
Student completed course and left the school, then came back within the same school year and was re-enrolled.
The class/classes are showing on the time table, but is not in the student's register.
Solution:
1) Drop the class from the timetable
2) Drop without Posting
3) Go back to the timetable and re-add the class to the student timetable.
4) Go to the Register and Edit the Start date from today’s date to the correct date.