Blank thumbnail slot

There seems to be an issue with the number of images that show up.
I have the widget set to show 9 thumbnails, and they rarely all show. I set all my thumbnails to the same size, but for some reason the widget doesn’t always fit all 9 images. A blank tile replaces the space/slot where the image is supposed to show, and it’s really random. Any ideas?
Usually after I hit refresh in my browser a few times, it will show correctly. (chrome, ie and safari on ipad)

https://www.ads-software.com/extend/plugins/advanced-random-posts-thumbnail-widget/