Locked Items are Still Editable

I am running this on my production site http://rpg-fusion.com where we run play-by-blog RP games. The games are run using editable nodes within books. When a player edits the game move/node, the message saying it is locked shows, but anyone else that has access to edit that content can click the edit button and it opens for them as well.

I thought Checkout module would lock the file to prevent anyone else from editing. I have tried disabling/uninstalling various modules that I thought might be conflicting. I had WYSIWYG installed but disabled that when I saw a report here that said there might be a conflict. That didn't do any good. I use the DIFF module as well.

Any ideas what might be causing the locks not to stick?

Thank you in advance.