The ability to develop and use models to explain phenomena is a key component of the "Next Generation Science Standards", and without examples of what modeling instruction looks like in the reality of classrooms, it will be difficult for us as a field to understand how to move forward in designing curricula that foreground the practice in ways that align with the epistemic commitments of modeling. In this article, we illustrate examples drawn from a model-based curriculum development project to problematize and bring to the fore issues and tensions we observed through the course of modeling instruction. In doing so, we argue that instruction that is model-based may not be actualizing modeling as an epistemic practice to support student sensemaking. We suggest that this kind of enactment may be a result of the tensions between viewing models as content to be learned and modeling as a scientific practice in which the end products are not known ahead of time. We discuss the implications of our analysis for teacher learning and curriculum development.