When you are done 90% with the chapter you write, move on to the next chapter. Leaving it unfinished means you can come back any time and revise it better at a later time. Coming back and forth between chapters is also a good strategy. The bottom line is never hanging on to one chapter forever. Sometimes "unfinished" is better than completed. At times, something clearer may pop up out of the blue when you are away from what you do.