In this Mixitup Gallery script with show=”8″, it will display all 8 photos from each category in ‘All’ order by the last photo added (Descending order)
So if you have 3 categories and for each category you have added 4 pictures in the order like below
1. 4 photos added for first category
2. 4 photos added for second category
3. 4 photos added for third category
It will show all 8 photo initially and all 4 photos for third/second category, but for first category it will show empty (please note DESC order).
So please add photos one by one for each category.
You can also use Post Type Order Plugin to reorder the photos so each category will display some images.