I'm actually having the same problem as well. Very annoying! As for attempting to mess around with a solution, I tried adding a new image, replacing and even removing an image from the target atlas to see if upon rebuilding the atlas that would fix the issue... no luck. If its helpful to know, I have another atlas with only 3 small images inside of it and the sprite selector is generating previews for that one just fine. It's just problems with my main UI atlas that is incapable of generating previews. Could this be an issue where a certain threshold of images passed and the sprite selector no longer generates previews?