You're using Cmd-0 (zero) right? That's the numeric zero, not a capital O. If you are using Cmd-0 and it's not working, the you may have to reset your preferences. See Trouble Shooting 101 at the top of the overview page.
It seems to work using the "0" on the top row of numbers, but not using the numeric keypad on the side of my Mac keyboard. I thought it was odd that one combination would work but not the other.
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