Problem posing is a critical component in teaching mathematical modelling; however, challenges persist in ensuring authenticity in the posed problems. This study aims to identify the essential factors contributing to students’ inability to pose authentic problems in modelling. Experimental teaching involving three prompts for 20 Grade 12 students in Japan showed that reflecting on the purpose of problem solving triggered students to recognize authentic problems in real-world scenarios. Moreover, it motivated students to establish assumptions, interpret data, and validate solutions based on the defined objectives. In future studies, these findings shall be extended to open problem posing in the initial stages of modelling.