Background: A perennial challenge of primary care quality improvement is to establish why interventions work in some circumstances, but not others. This study aimed to identify factors explaining variations in the impact on clinical practice of a facilitation led vascular health intervention in Australian family practice. Methods: Our mixed methods study was embedded within a cluster randomised controlled trial of a facilitation intervention designed to increase the uptake of evidence-based prevention of vascular disease in family practices. The study was set in four Australian states using eight of the study's 16 intervention practices. Facilitators worked with intervention practices to develop and implement improvements in preventive care informed by a vascular risk factor audit. We constructed case studies of each practice's "intervention narrative" from semi-structured interviews with clinicians, facilitators and other staff, practice observation, and document analysis of facilitator diaries. The intervention narratives were combined with pre- and post-intervention audit data to generate typologies of practice responses to the intervention. Results: We found substantial variability between practices in the changes made to vascular risk recording. Context (i.e. practice size), adaptive reserve (i.e. interpersonal relationships, manager and nurse involvement), and occasional data idiosyncrasies interacted to influence this variability. Conclusion: The findings emphasise the importance of tailoring facilitation interventions to practice size, clinician engagement and, critically, the organisation of, and relationships between, the members of the practice team. Trial registration: The trial was registered with the Australian and New Zealand Clinical Trials Registry (ANZCTR): ACTR N12612000578808 (29/5/2012). This trial registration is retrospective as our first patient returned their consent on the 21/5/2012. Patient recruitment was ongoing until 31/10/2012.