the quandry of the writer

When I am driving down the street or running errands, things pop into my head like:

The sheet spilled over the truck cab, waving its perpetual farewell like a giant lady’s handkerchief.

But then, when I get down to working on my novel, I get so focused on plot that it comes out more like this:

There was a big white sheet in the back of the truck in front of her, waving in the wind.

I like to tell myself that I will go back later and fill in the good descriptive writing. I hope I am not lying to myself.