Gah! Why can't I write?

So I signed up for the JD slash ficathon thing. Got my prompt and pretty quickly had an idea of the story I wanted to write. So, I started writing and soon the rest of the story developed in my mind and I had I'd say 90-95% of it all mapped out. So, why can't I write it? I'm getting close to the end and the real "slashy" part of it. Am I just that out of practice that I don't know how to write smut anymore? That's kind of depressing, really. Grr. And it's due next week! I will get it done, though. I WILL. I just need to make myself write it. *sigh*