Where did all my words go?

It seems as though while driving I am able to think of truly eloquent things to write about. I have whole paragraphs / stories written in my head, and then when I sit down to write I end up saying something like “not much happened today” – end of update. What is that all about? Where do all of my words go? Why can I not write like I think, and why can I not think while writing? Am I broken?