Synthesis problems, which are problems requiring the application of multiple concepts such as energy conservation and kinematics, are common in physics curricula, and improving students' skills in solving such problems is typically a key instructional goal. Despite the prevalence and importance of synthesis problems, many students struggle with them more than with their single-concept counterparts. In order to identify possible bottlenecks on this task, we asked students to solve a problem synthesizing two different topics (including energy, momentum, circular motion, and kinematics) as well as a pair of single-concept problems involving the individual components, varying their sequential order (synthesis + single-concept versus single-concept + synthesis).We found that students' primary difficulties arose not only from the deficiency in applying the individual concepts but also from the inability to recognize the relevance of both concepts, which in some cases may be caused by one dominant concept overshadowing the other.