There are times story ideas (concepts) come in to your mind and due to this, due to that you lost them. Story ideas are not things that live with us as our eyes, ear, hair or legs. They are like light that suddenly breaks into our heart, we need to take a grasp of them before they go off. And once they go, we may not be able to get them again. If we do, they may not be beautiful as they were when they first dropped.
As a writer, you should always be with writing material; paper, pen, or writing device. The moment an idea comes to you put it down that 'raw', as it flows, do not put off the fire by trying to fine tune it at that moment. When you know you have really gotten the concept like a tangible thing then you can fine tune it.
Someone might say, what if I am not with a writing material as at that moment? Open your heart to it and let it sink into your soul. Dwell on it until you know you have gotten it and try as much as you can not to get distracted.The moment you get a place to write it, write it down.
Please do not discard any story idea(concept) because you think it is not what people want, what people read, what had not been written on. Accept it, write it down and make sure no part of the story is lost. Every story idea has it own day, the best is yet to come.
Now that you have gotten it, begin to ruminates on it, then light will begin to break forth. More idea to build the small one in your heart will come up. Write them down as well. Keep pondering until you have that little idea become bold in your spirit with clearer shape, until you have something tangible you can mould out of it with creativity and innovation. Hurray! A story is born.