Preview icons

I like what I have seen so far and I enjoy the program. I just dont understand why some icons show a preview and others dont. What is the size limit for a preview to show up on an icon? Or if the icon doesnt show a preview does that mean the images is corrupt. But the person I am sharing these image with can see all the images so it cant be corrupt. Just a thought but love the product.

JJ Wooten about Adobe Creative Cloud, v2.4.1