Typically, the issue is either with the link, video settings or your browser. Are you getting an error that the video thumbnail can't be generated? Try clearing your browser's cache and cookies, shutting down the browser and restarting it. The link you use should be a straightforward YouTube or Vimeo URL, with nothing added, shortened or redirected. On both of these sites, hosted videos will include an option for sharing where you can grab the URL. I hope this helps clear up the issue, but please let me know! - Thanks!