In this study, we consider the experiences of a professional learning community (PLC) who focused on fostering growth mindset to improve learning in a lower-stream grade 9 mathematics course. Using a sociocultural theoretical lens and drawing on data from a two-year case study, we summarize the ways that PLC members fostered growth mindset and then more deeply explore the obstacles they encountered. Participants found that shifting from a fixed mindset to a growth mindset required changes to the ways they taught mathematics. Even with these changes, shifting mindsets about mathematics learning proved more challenging than they expected. PLC members shared the view that the process was impeded by: students' ingrained fixed mindsets, views of mathematics as a right or wrong subject, assessment practices focused on grading, the need to overcome their own fixed mindsets, and the streamed nature of the course. Through sustained collaboration, they found ways to begin to address these obstacles, with the notable exception of streaming. This study provides evidence of the ways streaming can inhibit the efficacy of a growth mindset initiative and offers suggestions for teachers, schools, and divisions planning to implement a growth mindset initiative to improve mathematics learning in similar contexts.