I have a Google Calendar that is private. Using the iFrame generated on the calendar site allows me to display it in an HTML placeholder, but it no longer shows on the displays. I get a blank instead of seeing the calendar. The calendar shows in the preview just fine.
The Preview shows the Calendar since you are likely logged in with that user account at the same time.
The Display isn't showing it since it isn't logged in with any user name and as a result can't show a private calendar. You'll need to set it up so that the calendar is public in order for Displays to view it.