When Command-Tab changes the menu bar to an application but shows no window, the app switch worked. The missing part is window presentation. Command-Tab selects an app, and that app may have a minimized window, a window on another Space, or no open document window at all.
Confirm the application is active
Look at the application name in the menu bar. Open its Window menu to see whether any document windows are listed. If none exist, use the app’s New Window, Open, or recent-item command instead of repeating Command-Tab.
If a window is minimized, inspect the Dock. If the application was hidden, activating it may reveal its windows differently from a minimized state. Do not use “hidden,” “minimized,” and “closed” interchangeably.
Inspect Spaces and full screen
Open Mission Control and review the Spaces bar. The app may have a window on another desktop or in a full-screen Space. A system setting can influence whether switching to an application also switches to a Space containing its open windows, so verify the current configuration before assuming a failure.
Avoid duplicate recovery windows
Do not create a new untitled document until you establish that the original window is closed. Check the Window menu, Dock, Mission Control, and recent documents in that order. If a third-party switcher is involved, verify whether it lists applications or individual windows and whether it excludes minimized items.
The key diagnostic is simple: app focus and window visibility are separate states. Fix the state that is missing instead of remapping a shortcut that already activated the intended app.