When i use hotkeys from Alt+1 to Alt+8, it makes the windows alert sound and does not register a bookmark (Same with shift keys for accessing them). Alt+9 works fine though, which can already be accessed from Windows -> Scene View Bookmarks.
I suspect the culprit is a hotkey conflict. The Shortcuts window shows which Scene View Bookmark hotkeys are also assigned to other commands. If necessary you can reassign the hotkeys from there.
Recent versions of Unity open a dialog when you use a hotkey assigned to multiple commands.