If you had network trouble that might be an issue i "soloed" prophecy earlier but the triumph for completing it in general didnt even unlock because i got weaseled about 3 seconds after i killed the echo. Still got the loot but still, annoyed me cause it took away the checkpoint so i couldn't even go back.

Did you leave and come back to checkpoint? Heard if you don't do it in a single setting it doesn't count.