Teachers' perceptions of students' capabilities are particularly important in efforts to support ambitious instructional reforms. In this paper, we explore one teacher's efforts to resolve conflicts and tensions as she engages with new practices associated with ambitious mathematics teaching. While many conflicts arose in the first year of the intervention, the teacher's linear inquiry stance, focused on prescribed solutions rather than action and reflection on problems of practice, meant that deficit framings of students remained largely unchallenged. Supporting the development of an inquiry stance that problematises current structures and practices, and enables systematic exploration of conflicts, is key to challenging teachers' perceptions.