Here are the most common issues when trying to coauthor a file:
1) Make sure you opened the file directly off Stratus, rather than accidentally downloading the file offline and opening a local copy. This happens with some browsers when clicking directly on the file name, rather than clicking the … button and then choosing “Open.”
2) Make sure all users have proper permissions to access the folder and file in question
3) Look for a yellow “Enable editing” button in Word when the file opens. You should also consider disabling “Protected View” in Word, which sometimes blocks files from coauthoring
4) Try opening directly from the browser, or copy the URL of the file and open directly from Word – you can get the direct URL to the file by clicking on the … button next to it in the browser.
5) Make sure the file is not inadvertently “checked out” and locked for editing.
6) Make sure all users are using Office 2010 or later, and that the file is a “.docx” not a “.doc” file