A checkbox with the following text is on the Course Details page.

Students can only participate in the course between these dates

It is closely followed by another piece of information:

When selected , the course is in a read-only state outside these dates. 

I had a course set up using course dates rather than the term dates. The end date was open so the course should have been available to teachers and students. However, the course was sitting in Past Enrolments. It wasn't clear to me from reading the text above that ticking the checkbox would solve this issue.     

I worked out  (through trial and error) that if the checkbox is checked,  it moves from past enrolments into the current enrolments area and becomes active (read-write).     

Now that I've worked it out, the text you provide makes more sense but I would like to suggest that you change it to make it clearer: 

For example, next to the check box: "Make the course active between your chosen course start/end dates". 

The small print could say "Outside these dates the course will be in read-only mode and move to past enrolments".