Thanks for the link.
The necessary code is automatically added to the page and this is occurring as expected on your site.
However, lightboxes (including this plugin) primarily work on links to image files (e.g. https://domain.com/image.jpg
). The reason the lightbox is not activating when the thumbnails on your test page are clicked is because they link to other WP pages, not images.
When inserting an image into a post, the “Link URL” (for example, in the image properties after uploading an image) needs to be set to “File URL”. This will link the thumbnail image to the actual image file that the lightbox can display.