I played in the may Comp and just used game sync and tried to participate in the now finished comp to try and unlock my box on 'Y', but it's still locked. All I had to do with my 'X' game was try to enter the comp after it was finished to unlock it on that game. What have I missed?
Here are the steps...
1. Turn on wifi/go to random matchups
2. Click on online competitions
3. Click "Cancel Parcipitation"
4. Battle box is unlocked
Hope I helped!