Lightbox not working on New Posts

If images added to new posts are not opening in the lightbox, the most common cause is that the images are not linked.

Without links, there is nothing for visitors to click on to launch the lightbox. See WordPress' official documentation on specifying links when inserting images:

• Classic Editor: Adding links to images.
• Block Editor: Adding links to image blocks (aka Gutenberg).

Tip: WordPress remembers your last setting when inserting an image from the media library. As a result, if you insert an image without a link, subsequent images will also not be linked unless you re-enable linking.

Note: Image linking is disabled by default since WordPress 4.4, so you may need to re-enable image linking after updating from an earlier version before you insert your images.

Once the links are added to the images, the linked images should be displayed in the lightbox as in your other posts.