When the “Skip existing alt text” function is enabled, the app leaves any image that already has alt text untouched and only generates alt text for images that are missing it. This protects the alt text you’ve already written or optimized prior to installing the Pixc: AI Auto Alt Text app.
If you’d rather refresh everything — including images that already have alt text — turn the option off before you optimize. This allows you to select images, including those that already have alt text.