@edeades

Two possible reasons:
  1. The order.php class hasn't been over-written on your web server so you are running the original code Zen Cart code and not the special code that manages stock at attribute level (a common problem with FTP clients that don't by default over-write files already on the server).
  2. You are running a version of the plugin designed to be compatible with version Zen Cart 1.3.8a, and not the version 1.3.6 that you are actually running. For that you would need version 4.3.1 of the mod.