Response A is better because it clearly explains that the diagram is not visible and gives helpful next steps for re-uploading it. Response B is also correct, but it is shorter and less helpful overall.
Response A takes the win here because it does a better job of spelling out that the diagram isn't showing up and walks you through what to do next if you want to share it again. Response B gets it right too, but it's more brief and doesn't give you as much useful guidance to work with.