The behavior to automatically load a directory index file (index.html) is purely a function of the web server itself. If it's not doing that, a few things come to mind. Most of this will be in linux/apache speak because that's what I'm most familiar with, but Windows/IIS will have equivalents.
[ul][li]Permissions on the index.html file itself. Depending on how it got there (real ftp, an upload script, an installation script, etc.) it's possible that the file can not be read by the web server process due to a permissions problem. Even if you can read it, doesn't neccessarily mean that the web server can.[/li]
[li].htaccess (or whatever equivalent) overriding the default directoryindex settings[/li]
[li]This folder is outside of the normal webserver's documentroot, so the default directoryindex directives do not apply.[/li]
[/ul]
That's what immediately comes to mind, there are probably other possibilities, that I'm not thinking of, but the problem is in the web server itself, not the application.