thumbnails not showing

Can you describe what you mean by broken?

Do the thumbnails appear normal in the post editor but not on the page?

What size setting are you choosing in the View?

It could be a theme conflict. What theme are you using?

If this is a live site, please post a link.

Thanks. I found no problem with the theme. Try this plugin: https://wordpress.org/plugins/regenerate-thumbnails/. If that doesn’t help, try removing and re-adding a featured image.