There are a number of reasons why you could be having trouble. Public records in the South generally and in Virginia in particular often disappeared in the Civil War. The online genealogical services like Ancestry also have a lot of sources, but not all of them. In this it's like a lot of research now- 90% searchable from your PC, the last 10% requiring actual libraries and collections. You might try going to the county where Ann Harrison lived and checking things like tax records, probated wills. If there's a local historical society, they could help: such groups typically spend a good bit of time dealing with genealogical questions , and you'd not only be the first person to ask about local family records but quite possibly not the first to ask about Ann Harrison. If you haven't yet done it, you might also check the Library of Virginia, which has a number of records of petitions to the VA Assembly and court cases.

There was also a pretty high mortality rate for this period, and it was quite common for people to lose husbands and wives at an early age, so you can't assume that someone who was married at age 27 would have the same partner at 32. That, coupled with the amazing lack of imagination these people had when it came to naming their children, means you could have a man named John Smith with two successive wives named Ann, a daughter named Ann, and a niece named Ann, and so a meager record stating "Ann Smith m. E. Randolph 1761 " might refer to his first wife and her deceased first husband, his widow's second husband, his daughter's husband, or his niece's husband. The family of James Rumsey, the inventor, seems to have had a rule that every generation needed an Edward, a Charles and a Mary, with the result that James' precise extended family is uncertain. Your family tree might end having to be similarly vague, in some places.

We've approved the question, but you may want to post to r/Genealogy as well for an answer more in line with what you're trying to do.