This is related to the grid feature.
If the grid includes an image not wrapped around a lightbox, you can’t click on the image to display the full-size.
Here is an example:
Maybe any images in a grid context should be converted to a lightbox regardless of the site settings. 
「いいね！」 5
Moin
2
Images are now lightboxed when they are at least 100 x 100 pixels.
「いいね！」 2