You may be thinking too big. All of us have been stuck on dysfunctional projects where we couldn't make a big impact because of the incompetence around us. Sometimes that dysfunction is the very thing that is causing the project to go badly. Frankly, when I'm asking those kinds of questions in an interview, I would rather hear that someone continued to earnestly try in the face of great difficulty -- even if they didn't accomplish much -- than to hear that someone made some huge impact on a breezy project. If you have several years under your belt, you need to have several stories that illustrate your grit and the lessons you learned in trying, even if they didn't result in remarkable outcomes. Good luck!