Command-backtick only cycles top-level windows of the frontmost application. If it appears to do nothing, first confirm that the app actually has two eligible windows and that you are pressing the grave-accent key produced by the current keyboard layout.
Verify the target and key
Activate the application from its menu bar or Dock. Open its Window menu and count the listed windows. Two browser tabs inside one window do not create two macOS windows.
The physical key above Tab on a US layout may produce a different character under another input source. Open Keyboard Viewer or temporarily test a known layout to identify the configured key. Do not assume the printed keycap equals the active character mapping.
Check the system assignment
Review System Settings for the shortcut assigned to moving focus to the next window. Another application or global shortcut utility may intercept the same chord. Change one assignment, test, and record the result before editing another tool.
Inspect window state
A minimized window, a full-screen window on another Space, or a nonstandard utility panel may not appear in the expected cycle. Use Mission Control, App Exposé, the Dock, and the app’s Window menu to locate it. If the document was closed, reopen it rather than troubleshooting a switch to an object that no longer exists.
If the shortcut works in one app but not another, the difference is probably application window exposure rather than a system-wide failure. Keep a visual or menu-based fallback instead of installing another global shortcut immediately.