One thing I've noticed is that over the months and even years of gameplay over a forum, the game itself can tend to bog down or even stall occasionally. Other than the need to inject new blood and perhaps devise the odd trick to separate players who've dropped out of posting so that the game doesn't stall for their return, there's also a need to turn up the heat sometimes.
Sometimes the players are genuinely distracted by real life issues, but sometimes they run out of things to do or their motivation sags or the tension just dissipates from the game. Often the best thing to do here is to turn up the heat and get things really cranking. Have a thug accost them or some cultists interrupt them or a rival group try to steal their information (or succeed) or draw in some kind of Mythos encounter or have them stumble across a big clue that all is not what it seems to turn the game on its head.
While forum games are slow, they do require pacing just as any form of entertainment media does, especially story-based ones. Take that to your advantage and inject some adrenaline whenever interest seems to be petering out.