I dont know what has happened? Suddenly I press Alt+Cmd+Space, on an iMac and the Zoom tool does not activate.
Maybe a different order? Space+Cmd+Alt works for me.
It won't work if you press Alt first. Cmd+Space will get you the Zoom[plus] tool, then add Alt to get Zoom[minus].
Unless changed, Cmd+Space will give you Spotlight search.
Space+Cmd the Zoom In tool.
It turns out that some keys on the keyboard are failing. Individually, they work, but simultaneously they don't. It has all the earmarks of a planned obsolescence of Apple keyboards. Thank you very much, partners.