Industrial symbiosis (IS) is a key paradigm for achieving circular economy among industrial firms. Achieving new IS projects often requires outside facilitation, and intermediaries can help solidify and expand existing IS networks. While various intermediary roles have been identified in the literature, less attention has been paid to the potential challenges that intermediaries might encounter in their activities. Based on the case of the national symbiosis network FISS, the Finnish Industrial Symbiosis system, this study investigates the dilemmas faced when organizing IS networks. It identifies openness and value demonstration dilemmas, which hinder intermediary-firm relations. It also identifies collaborative intermediation processes among intermediaries in seeking to uncover value, ensuring community embedding of new networks and selective integration of intermediation activities. These processes can help overcome the intermediation dilemmas. The results on the collaborative intermediation and its development over time contribute to research on facilitated IS and on intermediaries in sustainability transitions. For policy-makers, the study pinpoints the need for collaborative intermediation where both national and regional intermediaries are involved to ensure both economies of scale and flexibility.