I feel like I've been fighting this one for a long time on the Guild, and my answer is that a lot of things contribute to this.

At one point, I bowed out as a GM and handed over the reigns. That RP is still going. The players were very pumped and excited to do it and I was feeling overwhelmed and burned out. 

When I've had RP's go under (and I won't lie, that's a lot of them) it's usually either that I start it and get no reply. I start it, get replies and feel like I hit a wall, like the other person's post is hard for me to reply to. To be fair, in that particular case, I have not, in the past, reached out and tried to talk it out. In another case, they stop responding and I am left hanging. I can't speak for their reasons.

I think -all- of these problems stem from one essential item -- communication. as a GM, you have to keep the lines of communication constantly open, you have to keep talking about the plot and at times, help guide a frustrated player through writing their post. You can do this by brainstorming with them, helping them by providing pointers to hooks you left in your writing or to even leave a checklist of stuff that's happened for them to work off of. 

To be fair, for me, this is a work in progress. I am trying to become more consistent and long running in my RP's. I'm tired of saying, "Oh, players dropped." That's maybe 30% of it at best. And I think the reasons they drop have to do with communication.

Hopefully, someone with success in keeping an RP going will illuminate us on the technique involved. ;)