Dealing with irresponsiveness from far-away scientists When meeting some scientists in person, I've had good conversations with decisions upon interesting plans for research cooperation and personal visits. But then, subsequent contact by e-mail is difficult, where the other party either doesn't reply at all, or very slowly (several weeks +). However, when I then meet them again at a later date, they are still enthusiastic about the project (where the bulk of the work is mine) and a short in-person discussion makes more progress than the past months of (lack of) e-mail have. So it appears that a lack of interest in the project does not explain the lack of response. The project is scientifically interesting, fits with my PhD, and the visit enhances my chances of finding a post-doc after my PhD (it's a very good institute for my field).

I understand that many scientists are very busy and flooded by e-mail, but it feels sad to abandon a potentially interesting project just because communication by e-mail is not working well. I'm quite sure it would work if I were in the same building. What strategies exist to mitigate this problem? I can think of:


Try to ask a common acquaintance to poke him/her (my local colleague has a shared friend with him/her). Pro: Might get indirect feedback as to why the irresponsiveness. Con: involves 3rd party (may or may not be on-site) not necessarily involved.
Send reminder e-mails (did you see my e-mail from 20 February?). Pro: increases chances of being noticed. Con: may be considered annoying
Phone. Pro: hard to run away. Con: I don't like phoning and I don't like being phoned.
E-mail someone sitting in the neighbouring office. Pro: neighbour has small step to irresponsive scientist. Con: a bit strange to involve someone I don't even know
Give up on the project. Con: means no project and no visit.
Try to do the project without their input. Pro: no need for communication. Con: might waste a lot of time by trying known dead-ends that I'm not aware of. 


What other options exist? What advantages and disadvantages have I not thought of? What is wise to do?
First: that situation is common. Actually, that situation is very common. You listed a large number of options that come to mind, let's discuss them a bit:

For me, that's the best.

Reminders don't cost you much, but they can only go so far before you risk being annoying.

I would strongly advise phone or Skype. Don't get me wrong: I hate phone too. It has the inconveniences of both verbal communication (you need to reply fast, and have no time to smith your words precisely) and written communication (you don't see the other person's face and non-verbal indicators). However, it helps if people are unresponsive to email. Also, consider that maybe they like phone better than email (some people do!).
As I said, Skype (or any video-chatting tool) might be a useful alternative to phone. Some people hate it, some people love it, so be sure to ask if they would consider it.

No. You can ask a friend or common acquaintance to poke them, but don't use someone you don't know well. Unless it's their job, of course: if you communicate with a big boss or dean or whatever, it is fine to use his personal assistant or secretary to check up on things or remind him of deadlines.

and 6. It's your choice to make, not much we can say here.



To your (already good ideas) I would add an important one:
   7. Set yourselves (or give him) a goal and a deadline. On one occasion where you actually meet him, use his enthusiasm to set an (achievable) goal, possibly with an associated deadline: decide that you want to present this work at this occasion, or submit to a given special issue that has a deadline in 6 months, or want to have the project finished by September to hire a post-doc with funding from the John Doe Foundation, …
Whatever the deadline is, it will spur contributions from his side. The research environment puts people under a lot of pressure with bureaucracy and deadlines. If your project doesn't have any visible deadline, he will never get it done. So, create one, even if it's more of a pretext. Then use that deadline as a hammer!