We now know that certain cognitive functions and other specific brain activities may be located in a particular part of the brain but it is their interaction that maximizes thinking. In other words, thinking requires skills in critical analysis as well as creativity. When the synapses fire between the areas of the brain, thought happen.
Writing requires both critical and creative thinking skills. Your challenge is determining how to use your skills effectively. Do you think about the beginning and end before filling in the middle? Do you know the middle before deciding upon your thesis and conclusion? Do you spew words before refining? Do you outline before proceeding?
Draw upon different sides of your brain, especially when stuck. You may surprise yourself.