Daydreaming Trailer Style

I have brief visual images, short bits of dialogue, and a mood for a story (actually, several stories) that just pop into my mind. But that's it. I can't seem to take it further. It seems like every time I have a "new story idea," it's really more of a vague trailer for a movie, and I have a hard time stringing together the pieces. I want to tell an epic story, or even just a heartwarming story, and I can think of dramatic moments, but I have a hard time coming up with the beginning and most of the middle, before the big ending. Sigh.