User Sync Plugin not working for me

I have a main Wordpress install on one domain, and two additional installs on subdomains of that domain. I've gone through the steps of putting the plugin on all three installs, and using the main as master and the others synced with main. When i try to login to the subdomains using an ID I know to be on the main, it fails. Further (and I find this strange), I looked into the database, and the user tables are duplicated across the installs (i.e., inside each distinct prefix), so i can't imagine why i can't login.

Any help with this?

I'm running the latest version of Wordpress, 3.2.1