RP Decay, or the inevitable process by which an otherwise healthy roleplay slowly succumbs to a quiet, painful death due to lack of activity or interest, is a pronounced problem in our Roleplay Section here at Bulbagarden.
I, your humble correspondent, set out to investigate why. My journey led me to many dark, foreboding corners of the forums (well, actually just one: The Archives) and what I found was frankly shocking (not really, most of the causes were what everyone already suspected).
So here they are, the three root causes of RP Decay I uncovered whilst delving through the RPG Archival and poring over the multitude of failed roleplays - with a particular emphasis on roleplays being run by well-known, respected, and active GMs (to remove the obvious variable of GM competence):
1. Waiting for the GM, while the GM is waiting for you.
This seems to be the most common problem, though by a narrow margin. Essentially, the story is progressing along at a healthy rate, everyone is happy, but it's been a while since the GM has moved the plot along. The GM is waiting for the players to reach a certain point or accomplish a certain task before moving on, but s/he hasn't told them so, and the players feel it's time to move on - so they wait for the GM to post. And wait. And wait. And... post a little bit of idle conversation. And wait.
The GM checks the thread periodically, sometimes for days at a time, waiting for the players to actually DO something... all the while, the players are checking the thread periodically, waiting for the GM to actually DO something. Usually, the GM catches on eventually and moves ahead with the plot, but by then it's too late - everyone's lost interest.
The solution: Never let your players "wait." If you sense things slowing down, don't assume they'll pick back up again on their own - nip it in the bud and post something interesting for the players to react to.
2. Lazy roleplaying.
Note: This is as much a symptom of an uninteresting plot or poor player-screening as anything, so it's not all on the players' shoulders here.
It's not always the GM's job to move the plot forward, if the players never do anything to further the story then it's not much of a roleplay. Basically, the phenomena at issue here is a player-attitude, which tends to be contagious in a roleplay, that the GM is the only one allowed to make things happen, and players just react. You'll see a lot of idle conversation between characters (unaccompanied by actions), laughs, giggles, and gasps in response to GM posts (in-character, usually - and unaccompanied by actions), one or two sentence posts (which, I'm sorry, are almost never acceptable), etc.
Basically, the player-characters never do anything. Like, ever. They just talk about what's going on around them, never acting but always reacting - leaving it all up to the GM to basically write the entire story, while the characters, like scripted automatons... react.
Obviously, this makes things very boring very quickly for anyone looking for an actual roleplay experience (though it can be fun for the few who are just looking for a social chat-thread in-character). The players actually interested in roleplaying, who in a healthy roleplay collectively do just as much to drive the plot as the GM, will grow bored or irritated and eventually stop checking the thread, tired of reading the mindless drivel.
The solution: This one gets tricky. If someone just isn't an experience roleplayer, you might shoot them a PM and offer some advice on how to improve. If, on the other hand, they're usually more involved than this, it may be that your roleplay is just plain boring.
3. Rules without teeth.
See Also: Spineless GM
I cannot stress this enough: You must always, always, always clearly communicate the rules and their consequences in the Sign-Up. If you don't, then you'd better be very flexible because no one is going to follow them.
There will always be 'that one guy' who tests the boundaries in every roleplay to see whether or not you're willing to enforce your rules (express or implied), and if you allow them to run amok, they will. And others will follow them, while anyone hoping for a heavier hand from the GM will leave when they realize no one is really in charge. There were a few repeat offenders here, but I won't name them, so don't ask.
The solution: Basically, never let a player draw you into an OOC argument about anything - you're the GM. If the rule is worth enforcing, enforce it. And if it isn't worth enforcing, let it go.
Those were my findings. Feel free to discuss, or offer rebuttal.