If you're the GM in a large-scale rp which is stalling, one way of doing it is to have a catastrophic event happen. Like an earthquake, sudden invasion, or kidnapping. Another way is to time skip the roleplay to a few days or weeks in advance. If there are a few factions in the roleplay, have them meet-up under a truce. That gives sneaky characters time to plot. Basically, create plot.
NPCs. NPCs are underused in most rps, serving as decorations to enrich the environment, when they can be dragged in to the plot and killed off quite mercilessly. They can serve to add drama and change to the group dynamics. They can serve to reveal alliances by asking for the near impossible from the players. How they react to it reveals and enhances their characters.
Another way is to actively encourage roleplayers to have multiple, core characters. This allows them to play with another character while waiting for a response from another player. It also keeps interest up as they have invested much more time and effort than if they were to craft just one character. It also serves as 'insurance' in case the other player suddenly drops the ball; they have another character to play with.
As a GM, be MERCILESS. If you have invested a lot of time and effort in setting up a roleplay, ensure players have roughly the same level of writing. Of course, players have been known to step up to the plate when grouped with the more eloquent writers, so it's really up to individual GMs.
If you want a higher level of roleplayers, include certain phrases in the application form to see if they have read it, like "Type 'Elliquiy is the best.' at the end of your application if you have read this." It helps to screen out the more problematic applicants who don't read through the whole thing and will very likely not read your posts as well, creating multiple plot holes that suck our very souls in.
BE MERCILESS, when a player does not respond after a certain time period, include this in your application form, that you have the right to takeover their character and NPC them. I usually give players a week or so of MIA, then send them a PM. If after two weeks no response, I control their character. If in a main role, I continue playing with them. If they are not as essential, I write them off to the side so if/when the player returns, they can pick them up. Of course, if players tell you in advance, then it's fine, but encourage them to write their character into a smaller role.
Since players come and go, have a plot summary to allow new players to ease into the RP. Basically, communication is key here. The more you communicate, the more you become friends, the guiltier they feel about leaving the rp. Encourage players to ask for help if they have writer's block in the OOC thread.
Creative freedom - give players nearly free rein in the storyline and events. Once they feel like they own the roleplay, that it's their creation as well, they tend to stick around and be more interested. Leave things open-ended. That said, have a few over-arching storylines to serve as guidelines so they can pick and play around with whichever suits their interest.
Seriously, screw nice. You have to be merciless as a GM sometimes or else the whole rp dies a sad premature death, and the countless hours you and your players have invested go to waste. It doesn't mean you act like an asshole though, just that you take a tough stance to selecting players and encouraging the pace of the roleplay along.