Struggling with writer's block?

I'm in the process of writing up my thesis proposal. It has to be done by Friday and it doesn't seem to to be going at all smoothly.

I'm not quite sure why I'm struggling. I know what my topic is going to be. I know how I want to introduce my question. I know how I want to examine the problem; which arguments I want to use; and how I think I can round things out. Trouble is, I can't quite seem to get things written up to my satisfaction.