I think it's just a trial and error kind of thing. Usually I can tell if I'm going to click with someone during the plotting stage. If they're giving me feedback, contributing their own ideas and are genuinely excited to start the RP, then there's really nothing to worry about. If it's the opposite with someone then I tend not to bother because it will just die anyway and I don't want to waste my time. 

On the other hand, those partners who stopped replying could have good reasons. Maybe RL got in the way, or they're just forgetful. Have you messaged them? Communication is key.