Yeah, this is a known issue with the spider 550 PL bounties. It's happened to me as well. It seems to be caused by players not doing damage to the boss before it dies. When the boss dies, the adventure completes, but the bounty doesn't register, because you weren't part of the kill. Unfortunately, the adventure is completed so you can't go back in. The solution is to have someone who hasn't done the adventure bring you into the adventure to try again. My recommendation is to do this week's adventure alone, as it's not terribly hard this week. Have them bring you in, promote you to leader then they leave. It's how I solved it when it was the Wolf one with the shield you had to pop with the orbs.