Thumbnail in Video posts

Fantastic theme, and the support by so many ‘Resolved’ tags is equally incredible. I have a query: I’m trying to make video posts exactly like the post here: https://demo.acosmin.com/themes/justwrite/you-can-buy-pharrells-grammys-hat-for-10000-on-ebay/

I’m able to post the youtube video by simply pasting the link, but I never get the thumbnail on the main page, unlike your demo main page which has pharrell’s thumbnail.

What am I doing different?

Thanks