Context: Research has found that clinical assessments do not always accurately reflect medical student performance. Barriers to failing underperformance in students have been identified in other vocational settings. Is 'failure to fail' an issue for medical educators in the UK, and, if so, what are its determinants? Methods: We carried out a qualitative focus group study exploring the views of medical educators (general practitioners, hospital doctors and non-clinical tutors) from two different UK medical schools. To make sense of a potential multitude of factors impacting on failure to fail, we selected the integrative model of behavioural prediction to underpin our data collection and analysis. Results: Ten focus groups were carried out with 70 participants. Using both theory and data-driven framework analysis, we identified six main themes relevant to the integrative model of behavioural prediction. These are: tutor attitudes towards an individual student; tutor attitudes towards failing a student; normative beliefs and motivation to comply; efficacy beliefs (self-efficacy); skills and knowledge, and environmental constraints. Discussion: Many different factors impact on medical educators' failure to report underperformance in students. There are conflicts between these factors and the need to report competence accurately (i.e. duty to protect the public). Although some of the barriers identified are similar to those found in previous studies, using a theory-based approach added value in that it facilitated a richer exploration of failure to fail. Insights offered in this study will be used to plan a questionnaire study and subsequent intervention to support medical educators in accurately reporting underperformance in students.