Act 1 - Boss Not Spawning (King In The Mists)

While doing the Ominous Altars quest in Freythorn, Act 1, I can't get the King in The Mists to spawn after activating his altar.

After completing the prequisite 3 altars and activating the final fourth to summon him, a single mob wave will spawn after which the ritual will finish and I will be able to get a reward from the altar, however as the King never spawns I am unable to finish the quest and get his loot/spirit buff.

I have attempted to reset the instance three times by leaving the zone, logging out, and trying again after 30 minutes or so, and each time the exact same problem occurs.

Update - Logged back in to give it another go and the boss spawned normally. Didn't do anything differently to before so I'm afraid I cannot provide any insight into why he didn't do so before, but at least I have my loot.