Why is formative assessment -- a proven powerful instructional practice -- so elusive in classrooms? As a regional professional learning provider for several years, Amy Burton rarely observed the use of formative assessment, even though, when asked, teachers could define it -- a quick check, during instruction, of what all students understand so far about the learning objective. The lack of formative assessment was apparent among both veteran and novice teachers, even though novice teachers' recent university coursework included assessment for learning. To pursue the mystery of why formative assessment was so highly recommended but not practiced, Burton led an action research cohort as part of her dissertation research. She explored how teachers participating in a reflective learning community shifted their mental models of teaching and learning as a result of using formative assessment to inform implementation of professional learning for long-term change. In this article, Burton discusses the action research project and how she built teachers' use of learning-centered practice in order to implement the use of formative assessment methods.