Some templates or plugins have a ‘lazy load’ or ‘smart load’ feature where images are not loaded until they are visible on the page. This feature does not load images that are in a hidden overflow that is scrolled into view.
To fix this problem, you need to disable the lazy loading of images. Usually this can be done in the template or plugin paramaters.
If you want lazy load to work with the gallery, you need to contact the developer of the extension the includes lazy load, and ask them to support lazy loading of images that are in a overflow hidden html element, and are made visible with scrolling.