Most writers, at some time, will have stories they can’t finish. If you’re not doing this for money, or hope of it, you probably never will finish the story, not even for an audience. The huge number of unfinished stories at FanFiction.com tells us that, as do any number of people I’ve known with unfinished MSS piling up.
For railroad writers, having one scene stall is a real disaster: you can’t get past it to the rest of the novel!
Call it Ingraham’s Law: You must have a finished draft to sell, or you don’t have anything to sell. No editor will buy unfinished first novels, because you may never finish. (Amateurs are excused as usual, but you might like finishing, if you could.)
The basic reasons a first draft stalls (as opposed to lacking any time to finish it) are–
1) Boredom/sloth. This is too much like hard work. No one warned you that writing is sand-hogging for your mind. Now that you know how the story works out, you’ve had your fun with it and don’t need the actual completion.
2) Fear. You’re afraid to write those scenes, because they touch sore spots of yours, or someone will disapprove. If you finish, someone will expect you to do something with it. If you finish this, you’ll never have another idea and your fun of writing will be over.
3) Lack of Skill. You’re in over your technical head and don’t know how to pull this off. You can’t figure how this scene would be played to get where you need to go. You can’t make the writing approach your vision. You don’t like what you’ve written and wish you had a collaborator who could do more with the material you’ve built in your head.
4) Confusion/indecision. This is sometimes the result of any of the above, but it has other sources. Trying to write commercially can have publishers’ preferences at odds with your Muse. You may have been away from the work long enough you’ve lost track of the plot. You may simply be pulled along either of two possible stories.
5) Growth. You’ve outgrown this story. It’s too primitive, juvenile, or derivative, and now you see it. It makes you hairball, or at least cringe.