This paper reports from a single qualitative case study which investigated challenges that might arise in a geometric proof lesson involving generative interaction with diagrams. Data consists of video recordings from a 120 minute lesson, taught with a focus on geometric proof in a Malawian grade 11 secondary school classroom. A post-lesson interview was also conducted with the teacher soon after the lesson. The findings indicate that the teacher faced challenge of lack of time, hence he did not complete some planned activities. The students faced the challenge of lack of understanding of the problem. As a result, they were unable to devise a correct plan during their initial involvement in the generative activity. The findings suggest that both the teacher's and students' challenges would have been avoided if problem solving was conducted appropriately.