"No video thumbnail for this post."

So nothing shows up as the featured image or in the video thumbnail meta box in the post editor?

If those two tests are working, there’s a good chance your theme is storing the embed code in a custom field. Are you entering the embed code in a separate box from the main post content? If so, you’ll need to enter the name of the custom field in the settings page for Video Thumbnails.

To find the name of this field open up the post editor, click the “Screen Options” tab in the top right, then check “Custom Fields”. You should now see a “Custom Fields” section in the editor with a list of custom field names and values. The name of the field with your embed code is the name you’ll enter in the settings.

No, nothing shows up in post editors.

I checked the custom field.It says:
“External video code” and my code is embedded next to it.
I saved that to the custom field settings, but it still doesnt find it in my post.

Custom fields are usually named using a format like video_code, so I’m guessing “External video code” isn’t the actual name it’s being saved as.

Once enabling Custom Fields in the Screen Options tab, you should see a box like this. If you still can’t find it in that box, there’s a possibility your theme is using a hidden custom field (meaning the name begins with an underscore) and you’ll have to ask your theme’s developer for the name.

Thanks very much, I’ll email the theme developer.
Cheers

Great, let me know the result!

Did you get this resolved as I am having the same exact issue with my thumbnails?

Scratch that, went into settings, entered the custom field name and it did the trick!