I have a great start to a story; I've developed (or have notes on) terrific leads; I've got a fantastic ending.
HOWEVER, I'm stuck--stuck in the middle!
What happens in the middle?
What will my characters do to get from the beginning to the end?
What is the meat of my story?
Lots of writers get stuck. It helps to think of a story like a sandwich--maybe a turkey or a ham sandwich. Without that good meat in the middle, it would stink, wouldn't it? Well, if there's nuthin' good going on the middle of your story, it will stink, too. Just saying.
Whatever you do, make the middle the biggest part of your story.
Make the middle YUMMY!