High-quality preschool programs are heralded as an effective policy tool to promote the development and life-long wellbeing of children from low-income families. Yet evaluations of recent preschool programs produce puzzling findings, including negative impacts, and divergent, weaker results than were shown in demonstration programs implemented in the 1960s and 70s. We provide potential explanations for why modern evaluations of preschool programs have produced less positive and more mixed results, focusing on changes in counterfactual conditions and preschool instructional practices. We also address popular explanations such as subsequent low-quality schooling experiences that, we argue, do not appear to account for weakening program effectiveness. The field must take seriously the smaller positive, null, and negative impacts from modern programs and strive to understand why effects vary and how to boost program effectiveness through rigorous, longitudinal research.