Over the last 2 decades, youth-led protests have increased. However, whereas youth mobilization has been widely examined, why some youth participate while others remain on the sidelines has not been adequately explored and remains not well understood. In 2019, protests against an extradition bill amendment led to unprecedented mass demonstrations, riots, and electoral turnout in Hong Kong. Young people were among the largest and most engaged participant groups during the movement. Yet even though protest action became a part of daily life on campuses, some students who sympathized with the movement did not become active supporters. This analysis examined reasons for non-participation using survey and interview data from undergraduate students. In particular, this study analyzed possible causes for the non-conversion of sympathizers into participants and the erosion of protest participants. Differences among students were explored based on their origin. The findings showed that non-participation was not merely a result of the ineffective mobilization of otherwise highly sympathetic individuals. Instead, perceived (in)effectiveness, identity conflicts, and barriers played an important role in individuals' decisions not to participate in protest action. The findings further our understanding of non-participation and are discussed with respect to students' networked participation, the processual nature and individual agency in non-participation, and the implications for culturally diverse societies.