Boring Book Syndrome

Most of us (if not all) have had a rough draft that petered out and died. This is a separate form of writer's block. This is the Boring Book Syndrome. Symptoms include:

We've gotten bored with it,
We can't figure out what happens next, or
We're so in love with the idea and character that we refuse to let it go.

Bored
The idea may be interesting enough but even those can fall flat. If the writer isn't passionate about the idea, the book will come to a standstill.
Bestselling books don't always sound the greatest (Twilight: Girl falls in love with vampire. Harry Potter: Boy goes to wizard school.) And ideas that sound cool may never make a good book. (Black Powder: Boy goes back in time to stop the invention of gunpowder so his friend won't die. Have you ever heard of it? Probably not.)
The point is,if you as the writer aren't totally into your story, maybe it's not the one you should be writing (yet). Don't trash the idea completely; set it off to the side until you're ready to give it another shot.

What Next?
Don't freak out if you don't know for sure what happens next. Write whatever you're struggling with on a post-it-note and slap on your bedroom door. Then don't worry about it; you're subconscious will take care of the problem and one day the answer will come to you.
And if that doesn't work, rewind your plot back to the last point you felt the most energy. Un-write whatever comes after that.

Tough Love
Sometimes you love the characters and world you've created so much that you don't want to give up. But you can't go any farther and your plot is lacking the finer points of credibility.
Write it anyway. Fix it as you go along; revise. Some people say 'don't write it if it isn't a spectacular idea.' That really depends on the writer. Some of us are character-driven. Some of us live for plot. Don't dump it just because it isn't perfect. Rough drafts are never perfect.
If there's any reason for a story to be loved, that's a good enough reason.