If you're on the step where you want to place all the ultimate staffs in the robot head and the fire one in the main room, and you're on solo it won't spawn, or if you're on easy, it won't spawn. On two player it will. I think it's because one step requires one person to be near the cracked surface to call the airstrike in on while the other person needs to be in the head to push the button, so maybe it's locking you out to progress further?
Hey can you do the easter egg with only 2 people?
sorry for being off topic