Masonry doesn't work

I’ve added the following shortcode [cactus-masonry quality=”full” horizontal_spacing=”5″ vertical_spacing=”5″ border_color=”#fff” outer_border_color=”#fff” image_background_color=”#fff” show_lightbox=”true” show_lightbox_title=”true” browse_with_lightbox=”true” posts_per_page=”15″]

and for whatever reason the images are just stacked on top of one another instead of lining up masonry style.

https://wordpress.org/plugins/masonry-post-gallery/