The likelihood is that the photos haven’t been selected in the photo gallery widget.
In Site Builder, go to the Photo Gallery page and open the widget.
Click on the Photo & Video Gallery settings then click on Select Photos & Videos.
Filter by the required category or select Show next 30 until you see your ‘non-displayed’ photos.
Check the box underneath the images and make sure that you save and close the page to move on.
Save the widget and your photos should appear in the gallery.