Thank you for the details, I've looked up the issue via the back-end.
The problem is, that those thumbnails are not actually images, but the videos itself – therefore the plugin is not able to find any related image. However, the video link is stored within the 'qodef_post_video_link_meta' custom field, and it might be possible to link to the original youtube thumbnail based on the video key.
I've put together a custom code, that parses the video key based on the youtube URL, and links to the youtube thumbnail. Place it to the functions.php file in your theme/child theme directory (only copy from line 3):
IMPORTANT: Make sure to have a full site back-up before you make direct changes in your theme files. Also, I highly recommend using an FTP client to make the changes. The built in WordPress file editor will malfunction if a syntax error is found in the code.
Le me know if you need help with this.Best,
If you like my products, don't forget to rate them on codecanyon :)