This study aims to explore the challenges that pre-service mathematics teachers encountered in their first attempt at posing geometry modelling problems. The data collection involved the modelling problems posed by 34 pre-service middle school mathematics teachers who worked in pairs and their written reflections. The content and thematic analysis of the data revealed three challenges: (1) identifying a real-world problem that could be solved by school mathematics, (2) identifying a real-world problem that is open to multiple solutions, and (3) identifying a problem with multiple solutions, the efficiencies of which are considered for the real-world problem situation. These challenges suggest a well-planned training for teachers, which would help them to realize their struggles and provide the opportunity of improving their problem posing practice and understanding of modelling.