First Image as Featured Image not working?

I’ve installed and activated the Pro plugin, and entered my license key. In Bulk Edit I’ve told it to use the first image as the featured image, and I’ve set a category filter. On the next page, I’ve selected the category from the dropdown.

When I click Preview, it properly shows me the posts in that category. Because none of them have featured images (although all of them have first images within the post content, which are also in the Media Library), both the “Current” and “Future” Featured Images columns say that there is no image.

When I click the button to do the action, the list refreshes, but now it’s all in red with the word “Unchanged” and nothing in the featured image column.

I’ve tried different themes, disabled all plugins, etc., but it still doesn’t work. Help? Thank you! ??