In my opinion you need three solid things:


-A good setting and plot.

-An aim.

-Ability as a GM.


All three of these things are conducive to each other and I'll break down what I mean. What is your setting? Is it something original? If not is it at least something not being done much at the moment? There's no truly original ideas any more, but is your is your RP just another 'x' type of RP or does it have a hook. The forums can at times be saturated with samey RPs.
What are you trying to do? What's the aim; the core philosophy of the RP? Is plot central or is player interaction or perhaps combat/a sense of winning? Has this core ideal been conveyed to potential players?
Probably most important though is how good are you at actually selling the RP, getting players motivated and keeping things organised, essentially, your own abilities as a GM.

Perhaps share the specific roleplay and more particular feedback can be provided.