Forum:Underdome Mission problems(360)?

Just playing the The "Prove Yourself" mission on X-box 360. Got to 4th block round 4 of 5. Sniper Rifles came up on wheel. Appear to have killed everyone using the SMG. Mission has not completed almost like it doesn't recognise that all enemies are dead. Anyone else have this problem? Do I have to start from scratch agan? Any helpful suggestions would be appreciated.