A blank preview is not one failure. The switcher may lack the window entry entirely, or it may list the correct window but fail to obtain an image. Record which state you have before changing permissions.
If the window entry is missing
Confirm the target app has a real visible window, not only a menu-bar presence or closed document. Check the switcher’s app, Space, display, minimized, and hidden-window filters. Search by title if available and refresh its window list. A missing entry is usually a discovery or filtering question before it is an image question.
If the entry exists but the image is blank
Open System Settings > Privacy & Security > Screen & System Audio Recording and confirm the exact installed app is enabled. Follow macOS or the app’s instruction to reopen or refresh it. Test an ordinary non-sensitive window. Some minimized windows, protected content, or app-specific surfaces may not yield a current image; confirm that limitation with the relevant product or system source.
Avoid destructive resets first
Do not clear all privacy settings, preferences, or Keychain items as the first step. Record app and macOS versions, the affected window state, and whether focus still works. Wallo previews depend on current discovery, capture permission, and window state. Blank imagery does not by itself prove that Accessibility focus is broken or that more permissions will bypass protected content.