Thanks for reaching out to us!
Our screenshot server for the background of the preview is sometimes a bit overloaded with requests and could make it take quite a while for it to take a screenshot.
It should be fine now and you can see your site in the preview. I would like to also let you know that even if the screenshot of your site is not yet loaded in, you can still see the button in the preview and make live changes. 😄
Please let me know if you are able to see your site in the preview now! 😃