Image not show in Admin

Hello thanks for that precious plugin that help me a lot.
I’ve use it on a lot of media in my website, but for a curious reason, sometime some of them no more been visible inside the Admin. For instance when I clic on a media in the media list to show its detail, I don’t have the image showed anymore. Sometimes, it’s in the preview of the article (still in the admin)…

What could be the problem ?