Easy FancyBox initially scans the page source for media links on the “Document Loaded” event. This means right after the page source has become available to and read by the browser. When content is added or modified through AJAX (meaning after initial page load on user interaction like scroll or click) like Infinite Scroll by your theme or another plugin, then FancyBox will not be aware of any media links in that new content.

To make Easy FancyBox rescan the updated page source after content has been modified though AJAX, there is an event listener available. This event is also triggered by the Jetpack Infinite Scroll module. To use this event, you’ll need to modify the theme or other plugin script that handles the AJAX calls.

You can trigger the event like this:

jQuery(document.body).trigger('post-load');

Note: It completely depends on the AJAX script where this code snippet should be placed. Optimally, right after the DOM modification where content is added or modified. In most cases at the end of the AJAX Success handler.