I started a Score War and played the song right away but it's still saying I didn't play the song yet!
Rock Band Blitz needs to get information from the website before it knows that you're playing a Score War, and it has to know you're playing it before your score can count! After you begin a Score War, you need to connect to Rock Central on your console. If you're already logged in on Rock Band Blitz, just back out to the title screen and log in again. This will update your information, and now you can log a score so high your opponent will weep.