In this paper, we investigate the role of metacognitive postdiction skills, intrinsic motivation and prior proficiency in mathematics as Propensity factors within the opportunity-propensity (O-P) model of learning. We tested Belgian children from Grade 1 till 6 in January and June. The study revealed overlapping yet different predictors for mathematical accuracy and fluency, which led us to the practical recommendation for teachers to pay attention to both aspects of mathematics. The metacognitive postdiction skills of children were related to accuracy in mathematics during the whole elementary school period. In addition, we observed that children evaluated their own performance as worse when they were slower in Grades 3 and 4. Intrinsic motivation was related to accuracy but not to fluency in Grade 3. Especially prior mathematical accuracy mattered as a propensity factor. More than half of the variance in accuracy and less than one-fifth of the variance in fluency in January predicted the performances of children for mathematics in June, a finding that highlights the importance of longitudinal designs including students' prior mathematical accuracy' as well. Finally, we observed that poor mathematics performers are less intrinsically motivated, and less metacognitively accurate. Moreover, they overestimate their performances more often than well-performing peers in all grades, stressing the importance of paying attention to these aspects in mathematics education.