Great article Candi! :3 But I'd like to add one more if I may. In my years of forum role playing, the main reason of this:
is probably because there's no clear advancement in the story for him to continue. Thus another point I'd like to contribute is to advance the story in your post to prompt other player replies.

Advancing the story could be anything ranging from putting your own characters into trouble or passing the puck to another character that you have in mind. (i.e: tripping in the middle of combat in front of another player character, prompting him to act/post next). We need a carrot to bait other player's involvement and help each other out in sparking that idea for a post.