We've recently had issues where Excel sheets are no longer allowing co-authoring. When the user edits within Excel Online, it's fine (though there is a new step where the user opens it and still has to click "edit"). But from there, when the user opens to the Excel application to edit while remaining connected to the could and another user is also editing, the sheet becomes locked. Neither PPT or Word are acting like this and the problem is only persistent in Excel. When we rolled out 365, this wasn't happening and co-authoring whether Online or in the application, was fully functional.