Which type are you? Suzanne Lieurance, the working writer's coach at The Lieurance Group, added some helpful comments during her teleclass about "plot-driven" writers versus "character-driven" writers. She reported that she learned she is a plot driver, which means her writing tends to focus on what comes next, how it happens, and how to move the story along without involving much of the character's personal lives or emotions. Dan Brown and John Grisham are "plot" men. Romance writers are most likely to be character driven.
Do you have a hard time "letting it flow" when writing? If so, it could be why non-fiction may be your expertise. Most people tend to over edit and criticize their own work. If you edit, format, and document sources in each paragraph before going to the next one, this can make it difficult to flow with the plot and characters as a fiction writer.
These are just a couple of ideas to come from the teleclass.
Write it down