This grounded theory study seeks to 1) understand what barriers North Carolina kindergarten teachers experience as they implement a new authentic formative assessment, the North Carolina Early Learning Inventory (NC ELI), and 2) identify factors that increase implementation fidelity with the new measure. Data were collected from 10 teachers, from three regions in North Carolina, using semi-structured interviews and analyzed using constant comparative methods. Findings suggested that teachers have strong formative assessment practices, yet unclear expectations and insufficient training with the NC ELI contributed to poor implementation practices. The North Carolina Department of Public Instruction and school districts across the state should ensure adequate training and support to mitigate the gap between teachers' capacity for engaging in formative assessment and their use of the NC ELI.