Employees enrolled onto learning, but not appearing in offering assignment list

Summary

Learners are enrolled onto offerings, but are not visible

Content

I have a number of offerings where we've got learners enrolled, however I can't get their details to display on screen in order to change their status (either to withdrawn or completed).   I can see their names on the All Offering Assignments by Course report and the correct number of active assignments is showing in the Learner box on the offering page, however the individual records are missing from the list of users within the offering.

Why are some records hidden and how do I get them to show up?