The ideas students have about what causes math failure are known to impact their motivation. This dissertation throws light on how attributions of failure are negotiated during math tutoring at a non-profit STEM-based after-school program. The study employs methods of interaction analysis on a small number of cases to qualitatively document how tutor-student dyads co-construct stories about failure. The project addresses the theoretical relationship between students and tutors’ enacted responses to failure and their enacted knowledge-construction practices. I argue that responses to failure involve constructing obstacles, blaming causes of obstacles, and intervening to resolve obstacles, which take place as part of public practice. How students recruit sources of knowledge (perception, reasoning, introspection, memory, and testimony) in knowledge-construction practices, and how they tell stories about breakdowns in those practices are core concerns of the dissertation. By understanding the interactional mechanics of failure, the study can inform discourse-level interventions in the future.