What to do with a likely collaborator who is not answering mails? A few months ago, I contacted a person who conducted research in a topic that could easily be combined with my own work. 
For this reason, I sent him an e-mail exposing my research and why it could be interesting to collaborate, taking into account his results.
He answered me and showing interest in the work (that is what I read between lines). Furthermore, he recommended me some literature.

When I read about half of the documentation, I sent him a mail with the insights I got and several of my publications, justifying my points.
He answered me again, discussing some points and glad to have my work pending to read.

I finished his readings and I sent back my impressions. He continued the discussion and excused himself because of not having read my papers.

After two months, I have no reply from him. What should be my next step towards the collaboration?
Presumably your prospective collaborator expects something more tangible than email discussions to come forward soon. So far, what you describe sounds fairly typical - material and ideas are exchanged, and papers are read (or not). However, at some point this needs to move into a more concrete direction than "discussions". That is, you are now in the phase where the pedal needs to hit the metal - somebody, presumably you, now needs to come forward with a concrete action plan or at least steps towards such an action plan. 

Otherwise, if the discussions still remain on an abstract level even after multiple months and many emails being exchanged, I could understand that your collaborator fears that they may be wasting their time in this.

So the next step if you want to go forward with this would be to set up concrete next steps: arrange a planning call or even a visit, and then come up with concrete research methods and a methodology to address them. Particularly, be explicit about what you expect from the collaborator - the fact that they were not able to read your papers until now suggests they will be unable to contribute a lot of time, and it is better to clarify how they can contribute early rather than mid-way through the project.



I feel it is important to understand what the power and group dynamics are in such a collaboration. Why is everybody in the project? Who expects to get what out of it? Who is pushing / leading the project? Who has how much time to contribute, realistically?

These questions are all fairly different depending on whether you and your collaborator are students or senior academics. That is, if you as a student cold-mailed a senior professor the collaboration will ultimately have  different dynamics than if both of you are students, or both of you are similarly-senior academics.