Hi. That's curious. How the PowerPoint thumbnails are generated is by that the first slide image is used as the default for all the slides and then in the background each slide is exported and the thumbnail updated with the correct image. So if that export process stopped for some reason then it could explain why the last ones weren't changed. But that export process is reasonably robust so it could be a historic bug now fixed.
Can you please email email@example.com with details of the version of ZionWorx you're using and also the PowerPoint if still available. We can investigate further and come back to you.