This has been my experience as well. The first day the mission released, all the extra bosses spawned but because I didn't read the part where they all have to be done in one sitting I didn't get the tormentors to spawn. Every time I've tried the Landing after that, the boss won't spawn. The one for the Impasse did spawn again the next time I tried and I haven't tested the Blooming yet. Definitely frustrating. Edit: Okay... I went to test the Blooming and I saw a green light in the distance which is what the guides tell to look for. I went to it and found the tormenters and unlocked Dual Destiny. For you guys that aren't getting spawns check the Blooming and see if the green light shows up in the skyline for you. It may be that you have credit for the kills already. Note: I didn't do any of this in one sitting, this was over the course of days!