First off, thanks to Dave for becoming follower number three. :)
So you decide to start a game, great! You have some initial plots in mind, have created NPCs, you have amassed a tremendous amount of background information provided on your message board, mailing list or web site. You now proceed to advertise at a variety of message boards and mailing lists.
And...No one is interested. No bites, no queries, nothing. So, what is wrong? Why isn't your game generating any heat? Here may be some reasons.
Bad Idea: You might think your game is awesome, but it is possible it sucks. When coming up with a game it might be a good idea to field the concept to some friends. You might not always get an honest response but a little feedback never hurt. And it is not a bad idea to discuss the idea on game message boards to refine your concept.
One example of a bad idea that generated virtually no interest whatsoever was one I saw ten years or so back, when PBEM.com was still around. That was The Place to post your ad. There was a guy who kept advertising the same game concepts over and over again, where he would play this principle character and everyone else would play the supporting cast. It reminded me of the lame Indiana Jones RPG where one player was to play Indiana, and everyone else would play Sallah, Short Round, Marion, etc. I think you can imagine how popular that game was. This guy did not seem to get that players do not want to play subordinates to GMPCs...Well, let me clarify that. In Star Trek games most of them have the captain is run by the GM. Players do not want to play characters who are so obviously inferior to GMPCs. In a Star Trek game even if the ship is run by the GM's character their own have plenty of opportunities to shine. This guy did not seem to get that no one wanted to play a mere mortal in his super human's world.
In situations like this it might be a good idea to simply ask the advice of others. Ask on message boards and mailing lists if your idea is a viable one, and if they do not think so ask them why. Do not be afraid of constructive criticism and do not take any potential personal attacks personally. It is the internet, trolls abound.
Bad Ad: Presentation is key and if you are unable to produce a well written, coherent advertisement then likely no one is going to play your game. Spelling and grammar are still important, and a clear and exciting ad is needed to sell your game. Edit yourself carefully before you post your ad, and bear in mind that you do not need to spell out your game's every little detail. People often have short attention spans and if they see your ad is twelve paragraphs long they will likely skip most of it. Specific details can be addressed on your web site or you can answer questions fielded to you via e-mail. It also could not hurt to have a friend of yours proof read your ad as well.
An ad can be best served by a good, catchy title, one that can give a prospective player the gist of your game right off the bat. For example, for my Star Trek: Vixen game I believe I used the tag line "Star Trek meets Firefly" as it was about a merchant vessel and her crew in the 'Trek universe. Right away it suggests the theme of the game, and it might draw the interest of Firefly fans. In the body of the ad I fleshed out the concept a bit more.
Finally, what is very important is you provide the e-mail address you will be using associated with the game as well as the web site, mailing list or message board associated with the game.
Bad Reputation: And this leads me to how you are perceived by others. Your actions have consequences and if you have a reputation of being a poor role player or a troll, these things could haunt you. The gaming community is not all that large and word can get around. If you have proven to be a difficult person to get along with or have a reputation for being vindictive and cruel then these things could harm your chances for running a game of your own. Now, I suppose you could fix this by altering your name and e-mail address, which of course means your entire game and subsequent relationships and friendships with your players will all be based on a lie. But hey, if running a game is so important to you that you want to deceive your players in perpetuity, who am I to stop you?
So to sum up, make sure your concept is one that players want to invest their time and effort in, possibly tweak accordingly or scrap the idea altogether and start over. Make certain that your ad is presented in a coherent and exciting manner to generate interest. And above all else, keep in mind that the internet has a memory and your actions can have consequences down the line that can possibly sabotage a game before it starts.