You want high attendance, you need to build quality into your games. Plotlines are just a minor aspect of a game. Props (structures, fireworks, non-AEG weapons are your tangible aspects and are very important), themes/mood (WW2, Vietnam, Stargate) and players already interested/signedup (this suprisingly can be quite important to some) affects your turnout. Not saying that you havent already incorporated these but you may want to tweak some to create the perfect game where a lot of people would show up.
Also, the most memorable game I've been to had only a turnout of 18 or so but had many of the factors listed above (Meats Nam game, tied with BL2 nightgame)