I have found that some days I can write up a storm while others finding the words is a challenge. This made sense to me when I looked at the two with respect to left and right brain activity. Your left brain relates to language and your right brain to creativity. Thus, when actually writing, it’s the left and when you’re creating or imagining, it’s the right.
When you’re in the left and words flow, then sit at the computer and crank it out. When you’re stuck with the words, step away from the computer, do something mindless like taking a walk, going for a leisurely drive, or doing house or yardwork to let the story flow in your mind. Once you have a clear picture in your imagination where things are going, it’s a lot easier to get them written. However, creating and writing at the same time is often difficult. Recognize from which hemisphere you’re operating and yield to it accordingly.