Defining accurate use cases is a critical yet underexplored phase of a process mining project. This study investigates the challenges project teams face in identifying, selecting, and specifying process mining use cases. Through semi-structured interviews with 13 experienced process mining consultants from various industries, the research identifies key barriers to effective use case definition. The study highlights that key challenges are primarily related to stakeholder involvement and collaboration. These challenges include a heavy reliance on process experts, overcoming stakeholder resistance, convincing stakeholders of the tangible value of process mining, addressing differing perspectives on the same process, and dealing with variations in domain-specific terminology. Additionally, the findings emphasize the need for practical tools, such as a comprehensive knowledge bases, to support use case definition. We derive four main directions for future research: first developing frameworks for stakeholder onboarding and enablement, second exploring collaboration dynamics within process mining teams, third offering practical tool support for better guidance in the use case definition phase and fourth providing a comprehensive, consolidated knowledge base of process mining use cases. These insights provide a foundation for paving the way for more impactful process mining initiatives.