There comes a point in every story where the words stop coming. This can happen for a variety of reasons, but one of the most common (at least for me) happens due to the fact that your subconsciousness and your consciousness are fighting.
Put less esoterically: You, the writer, have an idea of how the story is supposed to go, but no matter how hard you try, you can't make the story go that way. Or, somehow you push through and do figure out how to make the story go that way, only to discover that everything seems off and your characters sound hollow and artificial.
The reason for this, I think, is because your subconsciousness is (somehow) keeping tabs on everything in the story. It knows that the thing your wakeful writer brain is trying to do just isn't going to work. It could be because you're betraying the character, or because something else has to happen before the scene you're trying to write, or because it's a minor detail you've forgotten about.
So the subconscious brain throws on the brakes, but, because it can't actually talk to you, it's kind of like taking writing advice from Lassie. "What's that girl? Timmy fell down a well? And there's no narrative reason for him to be there? Go and get a doctor and a semi-colon? Is that what you want me to do? Bark once for yes..."
I have been stymied any number of times on any number of projects and I have, through trial and error, managed to figure out a) that this is just something that happens and b) that when it does happen, I should take a step back and figure out what's wrong. Sometimes, this means abandoning works in progress for months or years as my brain tries to sort everything out.
To wit: People with good memories may remember that I was working on a story about a character named Grey the Mercenary. It's a story that's still alive in my mind. I'm excited to tell it and I'm hankering to get back to it, but every time I try, I just sort of peter out.
And it wasn't until relatively recently that I realized the reason that I was having so much trouble was that my conscious mind designed the city wrong. The first version of the city was under a glass dome on an inhospitable world. While that certainly makes for a cool visual, the setup made it impossible for me to layout the plot-relevant locations in such a way that made sense. It also made it very difficult for me to get the characters from point A to point B in a logical way.
I finally was able to get in touch with my subconscious mind and learned that the city would work better if it was on a mesa and not covered by a glass dome. That way I could have the spaceport, the old city, the new city, the ruins, the noble mansions, and the illirium mine all in a nice, compact, geographical area.
Yeah, so that's what it's like in my head, sometimes. If I could figure out how to get both of the brains working together better, I'd probably crank out stories a lot faster.