For one TV show there are no thumbnails

[fuuman]

For all my TV Shows there are episode thumbnails taking from the episode itself. I would describe it as a "screenshot" from a second of the episode. And that's how I want it to be. I checked the "use image form the episode as thumbnail" in the settings.

But for one single TV show that doesn't work. All episodes of that TV show just use the poster of the season for the episodes inside that season.

I cannot find any other setting relating to that topic. Do I miss something?

Thank you in advance.