I've had two players let me know that the bosses on some quests do not spawn all the time. Specifically, the 'Marching Dead, Cursed' on the Marching Dead quest and the 'Fat Rat' on the Rat Tower quest.
Also the boss of the Privvy quest is prone to not spawning too.
I didn't have this problem.
However on the privvy quest the boss indeed did not spawn untill i re-entered the area. That fixed it though.
Sounds similar to the pillars and disk quest where the transition triggers aren't going off for whatever reason and people need to re-enter to start the quest.
Seems sporadic. Had this once with the sandspider quest and once with the Zombie Marching Quest.
One of my characters, Dardo Patter, experienced the 'Purveyor of Flesh Most Moist' not spawning on the Docks Barrow quest last week. Re-entering the QA and running around a bit caused it to spawn. Other People in the Party were Evani Patter, Blarg the Wicked and a Dwarven wizard who's name escapes me.