Establishing and maintaining key mathematical practices in whole-class discussions, such as justifying claims, representing mathematical objects and making connections between the representations, is crucial to the development of students' mathematical competencies. The aim of this article is to investigate how and why the establishment of key mathematical practices in whole-class discussions varies in a teaching mathematics through problem-solving project. Analyses of transcribed video-recorded whole-class discussions result in the suggestion that the complexity of the mathematical problem itself as well as the complexity related to teaching the problem may contribute to more procedure-oriented practices instead of competency-oriented practices that create opportunities for the students to develop their mathematical competencies on a broad front. However, the results also suggest that the teaching of complex mathematical problems might develop the teacher's establishment and maintenance of key mathematical practices. Researchers initiating an intervention project hence have to consider both the students' and the teacher's learning trajectories, which might not always coincide. Other important aspects for the researcher to consider are discussed and pointed out as important areas for future research, such as how explicit introduction of appropriate frameworks may support teachers in learning to teach mathematics through problem solving.