Although we possess intuitions about pedagogy from early in life, adults commonly fail to teach effectively in real-world situations. Why might adults struggle in more complex teaching tasks? Here we develop a simple teaching task where adults fail to teach naïve learners, despite reporting high confidence that they taught effectively. Using a formal model of a rational teacher, we analyze the sources of our adult teachers’ failures. Our model-based analyses reveal that teachers successfully provided high-quality examples, but failed to address hypotheses that naïve learners find plausible. We validate these results in a second experiment, where we find that constraining learners’ hypothesis space increases their performance in the task. Our findings help bridge the gap between children’s teaching proficiency in constrained tasks, and adults’ teaching failures in more naturalistic tasks.