vertical thumbnail issue

Hi, I am having a particular issue when creating a gallery using template 1, I have an existing gallery that is functioning as desired with 3 thumbnails per row (9 thumbs per page) in a landscape orientation using template 1 (3×3 grid). However, I have created a second gallery using the exact same settings and my images are stacking one above each other in a vertical orientation, I have tried and tried using the same images from within the gallery that functions correctly but they just stack ontop of each other rather than 3 per row or a grid formation.

Any advice? Many thanks.

https://www.ads-software.com/plugins/simple-photo-gallery/