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;
- Make a temporary content change in one of the text fields in the course
- Save and preview the course
- Run a course Update > https://gomolearning.freshdesk.com/support/solutions/articles/11000037970-updating-a-course
- Refresh the Course Structure screen
Once this is complete the thumbnails should become visible.