Hey James,
How classes show under Previous Enrollments is governed by settings set by admins, and in some cases teachers at your school. Since teachers have some control that is why I'm guessing you are seeing some inconsistency. In what I would call a normal situation, a school has Term Dates set for Canvas and when it gets beyond the End Date for that Term, courses move into the Prior Enrollments section. However, courses also can have their own Start and End Dates which can override those term dates. Instructors can set these to be different if they choose (usually, depends on if admins allow it).
So those aspects above can contribute to when courses go into Prior Enrollment (also call Concluded) status, but there is one more important consideration. In each course an instructor can choose to restrict students from viewing a course before the start date, or after the end date as shown in the screenshot below.
So for the courses you are NOT seeing at all, my guess is that those instructors checked the box to "Restrict students from viewing this course after end date". From my experience as an admin, sometimes instructor consciously do this to protect the course content. They don't want students having access to past graded assignments or other content which they might share with other students currently taking the course. In other cases they didn't know what the boxes did and just thought it was something they should click.
I hope that helps you understand that a little better better in terms of why you might be seeing some differences. If you want to be able to see those past courses again which are missing it would probably require asking the instructor about it.
Rick
