New thumbnails for posts not generating/showing up

Hi there!

The thumbnail generated for each of my posts was working until about March, such as this post, and then stopped working. Newer posts don't seem to be generating a thumbnail, and when I try to manually add the Thumbnail: to the metadata at the beginning of posts, neither URLs nor paths to files work. I'm using the Magazine template, but I tried other templates and similarly they don't show any thumbnails for newer posts. Is there still a way to re-generate thumbnails for posts? I found a previous question about this, but the link doesn't work anymore.

This isn't a big issue, but the homepage looks much more engaging with images! Thank you. :)