Engaging students in mathematical argumentation supports language development and conceptual understanding because students use verbal and written language in coordination with mathematical representations to notice patterns, make claims, and create and revise explanations. This study adds to the limited research base on how teachers learn to facilitate discussions specifically aimed to develop students' capacities to engage in argumentation. We analyze teachers' sensemaking about supporting argumentation in the classroom through their participation in an adaptive professional development (PD) design. The PD was embedded in two elementary schools with upper grade teachers which enabled them to engage in iterative cycles of investigation and enactment within their own classrooms. Through collaborative lesson design and inquiry into students' experiences, teachers attended to the language students produced as they tried to formulate conjectures and produce explanations and the classroom participation structures that supported them. Our results suggest shifts in how teachers understood their role. Both cases speak to the complexities teachers face as they try to realize the recommendations for enhancing language learning and provide insight into what "rich" discourse means.