Have you ever loved reading a book so much that you didn't want to finish it? Sometimes, writing a book can inspire the same sort of feeling. Writers spend a ton of time inside the world they've creating, looking into the minds of different characters, and ending the book is a very bittersweet experience. Sometimes, maybe too bittersweet. Getting to the end can be so difficult, authors end up putting it off...and ruining the story.
All Good Things...
Once an author is really into the story and into their characters, the writing flows naturally. The dialogue style of a particular character, specific thought processes -- it all becomes second nature. Putting an end to all of that can be really heart-wrenching and difficult. Sometimes, authors may not even realize that they're having trouble getting to the end of the story...they just keep inventing more and more and more story.
Getting that deep into a particular story can create a one-way flow of creativity; the author's mind is so focused on these characters and this setting that other stories just aren't springing up. That's how intense focus and concentration work. But all good things must come to an end.
It's not always easy to face that fact. Fear of facing the end can create all sorts of bad decisions, like over-writing. It can also lead to missed deadlines. I had a lot of trouble getting to the end of my most recent book, the last in the Deck of Lies series. My writing slowed down dramatically; there were nights when it took hours to write half a page of text. Finally I realized that I was a little bit unwilling to face the end of the entire series. The minute I realized I was having the struggle, my writing got right back on pace.
Sometimes, you've just got to grit your teeth and get to the end. In some writers, the fear manifests itself as too much writing; others (like me) have trouble writing anything at all. When the fear gets into you, it might cause all sorts of things -- you might constantly re-think scenes, and go back to re-write them over and over again. You might find it impossible to focus, and wind up doing everything but writing.
If you find that you're just not on track with your book, check your emotions to see if you have a fear of reaching the end. It's a really common emotion among authors. Endings are often more difficult than beginnings, and there's a ton of work that follows every time you finish another novel. Once you face the fear, it'll probably go away.