Massive online participatory platforms are an essential tool for involving citizens in public decision-making on a large scale, both in terms of the number of participating citizens and their geographical distribution. However, engaging a sufficiently large number of citizens, as well as collecting adequate contributions, require special attention in the functionalities implemented by the platform. This paper empirically analyzes the existing flaws in participatory platforms and their impact on citizen participation. We focus specifically on the citizen consultation ``République Numérique” (Digital Republic) to identify issues arising from the interactions between users on the supporting platform. We chose this consultation because of the high number of contributors and contributions, and the various means of interaction it proposes. Through an analysis of the available data, we highlight that contributions tend to be concentrated around a small set of proposals and contributors. This leads us to formulate a number of recommendations for the design of participatory platforms regarding the management of contributions, from their organization to their presentation to users.