In this study, we investigated the development of apprentices’ professional knowledge and problem-solving skills in the second half of dual vocational education and training (VET). We analyzed (1) the average level of development, (2) interindividual differences in this development, and (3) the role of initial cognitive resources, school-leaving certificates, and sociodemographic background as covariates of the development. Our analyses were based on a cohort of automotive technician apprentices. Professional knowledge (n=473) and troubleshooting skills (n=322) were assessed towards the end of the second and third year of VET. Based on latent change score models, our analyses showed a moderate increase in professional knowledge but no significant change in troubleshooting skills. However, in both domains, there was a substantial variance in change indicating that apprentices differed in development. These differences were partly associated with the apprentices’ achievement at the end of the second year of VET. Furthermore, we found that initial cognitive resources at the beginning of VET partly explained differences in prior achievement and, to some extent, in subsequent development. There were no effects of school-leaving certificates and socioeconomic status beyond the effects of prior achievement and initial cognitive resources, but our analyses hinted at some disadvantages for apprentices with a migration background. In sum, our study reveals inconclusive findings on the effectiveness of dual VET programs in promoting learning, especially in the domain of problem-solving skills, and points out heterogeneity in learning that needs to be accounted for in VET practice to improve success for all apprentices.