Guides may label the pickups in a different order than the triumph. 8 and 9 are the usual suspects, as 9 is close to the entrance of the Gorgon Maze, and 8 is near the exit. If you have checked where guides say 11 is, please also check where the same guide says 10, and 12 are, as the 11th in the order that you encounter them may not necessarily be the 11th on the triumph. If they are not in any of the three spots, could you please screenshot?