Why some of the images and/or thumbnails cannot be optimized?
Sometimes, after running the bulk optimization (located on Media > Bulk ShortPixel) there are still some images and/or thumbnails that aren't optimized. It's easy to know, as ShortPixel will warn you once the optimization is finished:
Usually, the causes for this are:
- The actual images are listed in the Media Library (or database) but they do not actually exist on disk anymore. A quick solution to this problem is to use a plugin like Enable Media Replace to replace the missing images with new ones. Then ShortPixel should automatically detect the new image and optimize it along with its thumbnails.
- The images exist on disk but they are not publicly accessible. This usually happens when the permissions of your files are not correct (when changing hosting providers, for example). The solution is to change the permissions for all the files and folders needed on the new hosting provider. Usually this can be done easily by your hosting provider.
- The thumbnails used to exist in the past or the thumbnail sizes are defined in the WordPress settings but they do not exist on disk. They usually aren't needed anymore anyway as they were needed, for example, by an older theme employed by the website, a theme that isn't used anymore. The solution to this kind of problem is to use reGenerate Thumbnails Advanced to regenerate your thumbnails.
For other possible reasons, we'd recommend you to take a look at this article: None of my images were optimized, why is that?