To foster students' learning of critical-thinking skills, I previously introduced ill-structured problems to provide students opportunities to apply content knowledge and thinking skills. However, I noted that my third-year, life sciences students were not solving such problems effectively. Therefore, I used a grounded approach and conducted content analysis of students' forum discussions to understand their problem-solving approaches. The students worked in small groups using asynchronous, online discussion forums (AODFs) to discuss their approaches to solving an ill-structured problem posed. Each group submitted their solution to the problem in an essay. From my analysis of students' posts at AODFs, students seemed fairly competent in using domain-specific knowledge and certain domain-general skills in scientific argumentation. However, they lacked the ability to properly define the problem scope and, consequently, failed to address the problem adequately. The study illuminated students' challenges and provided me possible ways to plan relevant scaffolds in subsequent iterations of the activity.