I apologise for being unable to reply in French.
The capacity to take common political action depends on the capacity to build one of the following:
- long term trust between different actors
- actual solidarity (commonality of interests within the same "class")
- mercenary power deals between institutionally acting agents
Social actors who have recently started working together around a topic may have very different interests. Students may view anti-austerity measures from the perspective of service consumers, or recently activated ideological believers. Unionists may view anti-austerity measures from the perspective of unemployment fear, or long term ideological motivations. As they haven't worked together before, they lack trust. Trust can work in cross-class alliances, and within the working class. It takes time to build.
The difficulty of mobilising class actions, means that solidarity is rarely achieved in protest movements. Even if unionists believe students to be workers, the achievement of solidarity within a union is a great accomplishment. Achieving solidarity across industries is more difficult. Solidarity only really works amongst the working class, and it takes a very long time to build. In the case of student organisations, they flower quickly and fade quickly.
Finally, institutional deal making happens between organisations that have alienated memberships. People trade away bits of the dreams that motivate their members in order to force commonality to achieve a limited outcome. Quite often smaller groups resent this, particularly when they view the limited outcome to have been the institutional interest of another group all along. (Let's compromise: you implement my solution). These "splinter" groups then operate outside of what institutions have agreed. One common example of this is the "black block's" maintenance of its freedom of action in larger protests.
Coordinating actions requires a lot of meetings. These can often become dysfunctional, and fail to represent attempts to build trust or solidarity or make deals. The meeting meant to organise actions becomes the meeting that organises meetings. However, if the groups or people going to meetings are honest, direct, and take actions based on collective decision making, or awareness of how different groups will take separate action, trust or solidarity can be slowly built.