I hate this part...

So I have what I know is a good idea. I have good characters...characters who are so anxious for me to tell their tale that they've invaded my dreams. I have a plot, a conflict, settings, all the ingredients. What I don't have is the ability to focus. I keep wandering around in the idea but I can't nail it down. I can't get the ideas to settle enough to be happy putting words on the screen.
This is a good story to be told. It's an old concept with new twists. The characters are superb. Why won't my brain focus?