SiteShot uses a WordPress.org service to provide the screenshots of websites. When you view your directory, SiteShot senda a request to that service. That request enters a processing queue. If that processing queue is busy it can take a little time to get a thumbnail. In my experience though, usually the thumbnail is available within a few seconds and will appear in the next page load. Now, if a site that is being screenshotted is slow to respond the WordPress service it will move on and come back to it as to not hold up the processing queue. At least that is my best guess based on experience because I do not have any real insight on how they manage that processing queue.