I finished 'X Marks the Spot,' Didn't get Prishe. I had completed "Storms of Fate" and "Shadows of the Departed," and couldn't get her still. Finally, I flagged "Apocalypse Nigh" and completed the fight only, I didn't talk to Aldo or Gilgamesh, and was able to get her. So, it looks like any quests you have in Tavnazia that involves her, will stop you from getting her.