I did a reverse-search for your avatar image and found the website mindbodyhealthy.ca, I hope it’s the one you’re reporting this issue for.
I can confirm that the absence of the gallery in the blog page is not related to you changing the layout to full-width, no sidebar. What’s happening is that you are using the gallery post format for posts that lack a gallery shortcode. The image preview you were seeing previously was extracting the gallery shortcode from the post content, so if you removed it recently that is the reason for its disappearance. Adding them back will make the gallery appear back in the blog.
If you would like the gallery to appear in the blog but not on single posts, you can hide it with this custom CSS:
.single-format-gallery . gallery {
display: none;
}
By the way, your website loads extremely slow. I am currently viewing it from London, UK and it takes almost a minute for any page to load.