Underdome: enemies not spawning?

[El Lele]

I was doing the Prove Yourself mission, and did Hellburbia with no issues. I was on wave 4 of round 4 in The Gully, killed every enemy but the round didn't end. The map didn't show anything either. I searched everywhere in the arena and there was no one left. Is this a known problem? Can it be fixed? I had to kill myself in order to finish it. If this is gonna be a persistent thing, It would be extremely annoying, not to mention too time consuming, specially in the longer challenges.

[Mister Torgue Flexington]

i'm actually planning on waiting until the first week of December (my vacation week) to run through the Underdome DLC, as I want a full 4-team with one of them being a lower-leveled character (Level-47 Soldier here) for those like me, who've never played through it before.