The truth is with interest checks on the Roleplayer Guild is the genre, the category, and then when it is posted that matters the most. First, the most popular topics tend to be of the fandoms, fantasy, superhero, school, or anime variety, particularly in the casual section, but free as well; the other sections narrow your audience greatly. Lastly, there is a general cycle of activity that flows with the current of the education system and colleges, meaning activity dips particularly low during the usual exam periods.

This one should try to keep in mind, but then comes the topic itself. Players tend toward a topic that is easy to read, familiar, with maybe a few paragraphs and then bullet point style details. A long introduction, higher reading level, excess fluff or detail, or lack of what you specifically want or do not want makes it less likely to receive the attention it might well deserve; think of it as an advertisement for a product, almost. 

Last, though not least at all, you might need to "bump" the topic per day or so as prospective members tend to look for and through the first few pages more than anything else. This all said, there is a bit of a game involved and hopefully this insight will see you further off than where you were.