Same issue here... also tried bringing in a sixth corpse with no sign of the quest completing.
There is also no indication in my journal that he has recieved any corpses from me, but that might be as intended.
I think I found the reason for this and it should be a simple fix. What I'm not sure about right now is if I have all the necessary files needed to rebuild the script(s) of if I need to pull orth out of stasis.
Why do I have an image in my head about Underworld concerning the vampire lords all of the sudden.
Don't worry about it...