Hello,
Sorry you are having problems with thumbnails not working. I would like to help. You say you attached the .json files. I don’t see them in the post. I only see a PNG. If you can send the GeoJSON files that would be very helpful. (URL’s would work, if they came from a public site.) Alternatively, you could share your CODAP document. (Click on the menu in the upper left of the page, then click on “Share…”, then follow the dialog to create a shared link.)
Thanks,
Jonathan Sandoe