Shifts towards student-centred pedagogies require high-quality professional development to support teacher learning. Although many characteristics of effective professional development have been identified, such programmes still struggle to demonstrate successful results. This mixed-methods study investigated the development of student-centred practices in middle-school teachers in the USA in response to a programme of professional development. Participants engaged in individualized programmes of professional development built around personal action research projects. Quantitative results showed a significant decline in student-centred practices, and explanatory qualitative data indicated differences in the perception of the focus of the professional development and engagement with dissemination. Mixed analyses indicated previously unobserved similarities across participants, highlighting the difficulty of measuring the impact of professional development using only quantitative or qualitative approaches. The impact of this professional development and implications for the study of similar programmes are discussed.