The Diablo Station Incursion sometimes gets stuck on stage two, when an enemy falls through the world or never spawns (or something). This is the second time I have seen this, this week: Once before the patch, and again after. Happened tonight at 11:00 PM, right around the 11:00 minute mark.
Also, that incursion tends to have a habit of mobs getting stuck in the factory walls, where it is difficult to kill them, and sometimes spawns are staggered with an unusually long delay between single mobs spawns (happens around wave 7 usually), which slows down the rate at which the siege can be progressed.
If you cannot ensure the mobs will spawn at the correct time, in the correct place, you should consider putting the timers back. For that one siege if nothing else.