Without actually looking your games I'll answer this on general scale.
Players need a personal attachment.
A hook that connects them individually into the game. In usual multiplayer forum RPGs it's the player character. Of course, having a character is not enough, there needs to be activity to come back for, a decision to make. Providing that
activity is the most challenging task. How to do that? Well, I'm not entirely sure. You need to provide two goals; short term tangible goal that players can reasonably archieve in relatively short time frame, perhaps within ten turns. Perhaps make it possible for players to visualize future short term goals. "If I can do this, then afterwards I can do this and that." Short term goals string together into path to long term goal that has major impact into the game or game world.
This is what I did with my RTD that somehow ended up in RTD Hall of Fame. Of course I wasn't aware I was doing it at the time, but in retrospect it is clear. The end goal of the game was crystal clear. Each player could easily see individual steps required to reach that goal and plan accordingly. All I needed to do was to throw some randomness into their plans and open new alternative paths. Sometimes those new paths were used, sometimes they were ignored. Once the ball started rolling, it pretty much rolled by itself and all I had to do was to punish my keyboard. It was great, but also very exhausting.
For suggestion games this is doubly more important because players do not have their own character. Short term goals and long term goals is far more troubling because those tend to be goals of the character rather than player's. I have ran only one suggestion game, and comparing it to more succesfull ones I have reached this conclusion:
The hook must be personal interaction. If GM doesn't address unused suggestions it will leave players without connection. By addressing those, answering them individually from context of the character, even if they are not used, you provide players the connection.
I have one suggestion game roughly planned (
which I will never run) to make players as voices in the character's head, so I can answer each individually from context of the game. For example:
Murder them all!!
No god damn way, I'm not a murderer! I won't give in to the dark side!And few turns later:
I told you to kill them. You wouldn't be in this trouble if you did.
LALALALALAA! I'M NOT LISTENING!Without trying this I can't be certain if it really works, if that's enough to give people the personal connection and desire to keep posting suggestions, but currently I'd like to think so.