For some time now I've noticed that if I accidentally go off and leave an Excel workbook open on my laptop or Surface tablet, when I open the app from my phone and vice versa I get a warning that the file is Read Only and that I have locked it? Now pardon me, but I thought that the whole idea behind Office 365, and Office Mobile was so that one could have people working on the same file at once.

On the tool bar under "Review" I noticed "Share Workbook" I thought AH HAH! but to my dismay when I checked that and opened the workbook, I wasn't even able to open it anywhere else. So much for being able to share......

What am I missing?

Thanks and Happy New Year

Windows 10 Laptop - Office 365 (2016)
Windows 10 Mobile - Office Mobile
Windows Surface RT Office 2013