Social movement activists frequently find it difficult to get included in local participatory institutions. After engaging, some researchers point to positive learning (i.e., more information or transparency regarding non-organized people); other works highlight negative outcomes such as co-optation or moderation of aims (Baiocchi et al., 2011; Lima, 2019; 2020). In this paper, we explore the effects that the engagement in participatory institutions provoke on social movement activists. Specifically, we address the problem of disappointment, which is analysed departing from a case-study of local forums created in Madrid in 2016-2019. Members of social movement organizations decided to engage and, soon after, faced overload and disappointment. This case is useful to understand why local participatory institutions tend to produce these feelings, and how they can be addressed.