When a Course is duplicated the thumbnails for the topics do not sometimes populate.

If this is the the case they can be restored by taking the following steps;

  1. Make a temporary content change in one of the text fields in the course
  2. Save and preview the course
  3. Run a course Update > https://gomolearning.freshdesk.com/support/solutions/articles/11000037970-updating-a-course 
  4. Refresh the Course Structure screen

Once this is complete the thumbnails should become visible.