Being able to represent physics problems and concepts in multiple ways for qualitative reasoning and problem solving is a scientific ability we want our students to develop. These representations can include but are not limited to words, diagrams, equations, graphs, and sketches. Physics education literature indicates that using multiple representations is beneficial for student understanding of physics ideas and for problem solving. To find out why and how students use different representations for problem solving, we conducted a case study of six students during the second semester of a two-semester introductory physics course. These students varied both in their use of representations and in their physics background. This case study helps us understand how students' use or lack of use of representations relates to their ability to solve problems.