Due to changes in Dutch mathematics education, teachers are expected to use new teaching methods such as enquiry-based teaching. In this study, we investigate how teachers design, implement and evaluate new methods for statistics teaching for 7th-graders during a professional development trajectory based on peer collaboration. We monitored teachers' development in a network of four mathematics teachers from the same school. By using a mixed-methods approach in which we combined data from interviews, concept maps and classroom observations, we describe changes in teachers' practical knowledge. We found how the nature of these changes highly depends on teachers' personal concerns that emerge during the trajectory. Some teachers considered their concerns as challenges stimulating their learning, while other teachers experienced their concerns as a reason to fall back to previous teaching methods. Based on our results, we give some recommendations for organising teacher networks.