This has to do with initial excitement. Once it wears off, you lose interest. You can prevent it by writing down all of your ideas, leave them for a week and then return to them and see if you are still interested. Work with the ideas a bit more and then leave them for a week again. I've recently taken to this method, which has made me scrap dozens of ideas. There is but one idea that I'm still interested in and still working on. When I finally get around to posting it, my initial excitement will be at a zero, which will help me to be consistent in running the roleplay.