A cognitive diagnostic model uses information from educational experts to describe the relationships between item performances and posited proficiencies. When the cognitive relationships can be described using a fully Bayesian model, Bayesian model checking procedures become available. Checking models tied to cognitive theory of the domains provides feedback to educators about the underlying cognitive theory. This article suggests a number of graphics and statistics for diagnosing problems with cognitive diagnostic models expressed as Bayesian networks. The suggested diagnostics allow the authors to identify the inadequacy of an earlier cognitive diagnostic model and to hypothesize an improved model that provides better fit to the data. (Contains 4 figures and 3 tables.)