The ability to choose problem solving strategies flexibly and adaptively is an important part of expertise. However, it is unclear how simple forms of problem solving practice affect flexibility. We investigate to what extent flexibility in strategy use is dependent on learners’ characteristics as intelligence, working memory and prior knowledge and to what extent these individual differences influence how a student can exploit a learning situation. In a microgenetic design with 24 trials of a mathematical problem solving task, we found that ninth-graders adaptivity of strategy choices increased linearly during practice without feedback and that feedback on strategy adaptivity facilitated the process. Adapting strategy choices to problem types led to shorter solution paths, higher solution rates, and higher speed. This is true independently of students’ intelligence or working memory capacity. However, prior knowledge was a predictor of adaptivity, thus leading to a faster development of adaptivity.