Arrange All windows

I'm using office 365 for mac.   when i open multiple spreadsheets and then go to "arrange all"  that option is grayed out.  if i start over and open two without making any sudden movements, the option may or may not stay available.  what is triggering it to turn off?  the two spreadsheets are open (using file open)  and i can click and go between the two.  but something.... is turning off the option to arrange.  any ideas?  have used on windows for decades and never had this issue.