The aim of this paper is to describe and analyze how a group of prospective teachers create problems to develop proportional reasoning either freely or from a given situation across different contexts, and the difficulties they encounter. Additionally, it identifies their beliefs about what constitutes a good problem and assesses whether these beliefs are reflected in their problem creation. This is a descriptive-qualitative study that utilizes theoretical and methodological tools from the Onto-semiotic Approach in the content analysis of participants' responses. The results indicate that the prospective teachers' beliefs about what makes a good problem do not always manifest in their practice. The prospective teachers faced challenges in inventing problems that meet the established didactic-mathematical purpose, related to insufficient didactic-mathematical knowledge of proportional reasoning, achieving better outcomes in the arithmetic context and in free creation.