I'm having a similar issue with drawing.
Sit down with all my tools and an idea in my head, I can even see the lines of the paper forming in my head.
But when I lift my hand to the paper/drawing tablet, I just... can't. I hit almost a physical wall in my brain, and I've been bashing my head against that wall for a long time.
You should pay yourself.
No, seriously. Find some type of reward or incentive you want (a candy bar, time playing a new video game, an episode of your favorite show, whatever) and say to yourself "okay, if I get X done, I'll get my reward." If that is getting a rough design of your drawing down on paper or writing 100 words of your next chapter, it will help get your brain moving. Just keep incentivizing yourself, little by little, and soon enough your project will begin forming.
And by project, I mean building a giant robot.