When you read through your research notes, summaries, commentaries, etc. is it always clear to whom you are referring when you use the word "she," "he," "they," etc.? Pronouns are great, but if you are writing about several people and then starting using "she" or "he" are the references clear from the context? If not, consider re-writing or re-phrasing.
Thomas Smith and Henry Johnson arrived in Colusa County, California, in 1856. Then he married one of the daughters of Jackson Brown and they moved to Oregon.
Who got married to the daughter of Jackson Brown?
It's not clear, is it?