Had some what samething happen to me in this fight.

The Duty never started as soon as we entered the dungeon. This may of happened because of the person that entered you in or 8th member of the party wasnt inside the dungeon to start off the fight. This means he got inside and disconnected and started the fight with 7 people or it still didnt register that 8 players were inside.

Once the duty didnt start Hydra was unable to summon any Wyverns. As soon as Hydra was defeated we were waitting on 8th Member to come back into the game. As soon as he was back into the Dungeon it said. Duty Started--> Duty Completed and 1 Wyvern spawned and we exited.

In your case game didnt register that 8th member back into the dungeon and were unable to complete the duty.