Adopting new instructional materials is an important way to support reform in science education, but implementation can be challenging and complex. Therefore, we conducted a contrasting case study of two middle schools implementing new curricular materials. We conducted semi-structured interviews with teachers and leaders and collected instructional artifacts and reflections from teachers. Using an organizational sensemaking framework, we investigated the tensions that came up during implementation, the resources leveraged to address those tensions, and resulting instructional and leadership practices. One school focused on implementing the materials with fidelity, relying on networking and past practices, which led to a more traditionalized teaching approach. The other school centered the curricular materials, prioritizing understanding the instructional model and customizing for their students, resulting in more aligned instructional practices. Looking across the two cases highlights three key takeaways. First, we discuss the dilemma of agency, which is how can leaders balance teachers' professional agency with the push for instructional reform, especially when teachers may not fully understand the reform approach. We propose that a focus on understanding the goals of instructional reform might help resolve this dilemma. Second, we discuss the role of collaboration as potentially supporting or inhibiting teacher learning and propose that when teachers collaborate about high quality instructional materials they may be more likely to learn about the reforms inherent in them. Finally, we highlight the importance of discipline-specific leadership in helping teachers to understand and implement the complexities inherent in new science instructional materials.