Horde mode XL wave never starting?

After beating wave 32, we ended up in a state where the wave never started. all players had the same reach objective, but the game never actually started wave 33. It was a very dissapointing end to a nearly 5 hour run.
We tried running around the entire map killiing any stuck / respawning mobs with no luck, and the objective pointer did move always trying to get us to the same point. Up until this point the game was running slower and slower, not framerate wise, but zombies moved slower, actions like using a medkit started to take longer than the use bar. Has anyone else seen this and found a way out of it? We ended up having to force-end the game through the use of grenades in order to get out.