Hi Jon,
Based on the screenshot, it looks like sorting on the course start date is causing the issue. Can you click on the reset sorting button on top of the overview page and see if it will give you all the results again?
But in order to fix the issue, I need more information about this specific course start column. Is this a custom field column? Can you send me a screenshot of your column settings to support@admincolumns.com? And do you have the same issue when you sort on other columns as well?