In 2019, Poland experienced a massive teachers' strike that lasted three weeks. Although the strike had a great impact on many teachers, in the end, this three-week action failed to obtain any of its demands. Using the concepts of workers' associational power and union legitimacy, I argue that one of the reasons for the loss was a crisis of legitimacy within the main union that organised the strike. Based on interviews with the strike participants, I analyse the dynamics of changing workers' associational power during the strike due to a crisis of legitimacy of the strike organiser -- the Polish Teachers' Union (ZNP). I show how the union faced a legitimacy crisis in four critical areas: perceived injustice, perceived effectiveness, union availability, and internal democracy both before and during the strike, and discuss how these insights might benefit trade union renewal.