The video thumbnail is not being displayed.

After a while since creating a video, I've had a recurring issue where the thumbnail on my profile page turns completely white and disappears. (Currently, thumbnails for three videos have vanished.) While I can still watch the video itself by clicking on it, I can't tell what video it is at a glance. It also looks quite bad when I put videos into an album, so I would like this to be fixed. Thank you.