I did this yesterday on my fighter/rogue and the Giant Sludge Thing didn't spawn. Abandoned the quest, tried it with my Cleric and it worked just fine.
Furthermore, I noticed that upon turning in the quest, the conclusion journal entry didn't properly go into the Finished Quests tab.
I don't see why the monster isn't respawning on this quest, assuming it still isn't. If it doesn't spawn, sometimes you may need to re-enter the area due to lag - but anyway, if there are further issues, report them please.