Videos not showing 1st frame when listed

Usually, when a video on Peertube is listed and not playing, its thumbnail screen shows an image of the first frame.

On my videos, I am just getting a blank image:


The video plays fine, but shows no thumbnail. I know it’s possible to add a thumbnail image manually to the video, but if I do that, that image then appears on top of the video.

Why are my video first frames not appearing in thumbnails?

Thanks for any help.