Hi Andrew
Did you use the steps in the following article to check out a file？
If not, please refer the steps above to test the issue. Before editing the file, please make sure the icon next to the checked-out file name is partially covered by a green, downward-pointing arrow , which means you have checked out this file successfully. When you hover over the icon of any checked-out file, a tool tip displays the name of the document and the name of the person who has it checked out.
If you have confirmed the steps and issue still exist, please provide below information to find the probably causes:
1. When you edit a file, are you using browser or Microsoft Word to open it?
2. Did you receive an error message when saving a file that has been checked out? What are the exactly words? Please post the error screenshot here.
If you are using Microsoft Word, please make sure you are logging in Office Professional Plus as the same user account with the one who check out the file.
Best regards,
Nancy Xie