Greg, the listing of people that are available when saving in Ancestry doesn't appear to be "Live" - it looks like there is an index of people that are stored in a "Cookie" in your web browser. That index may not always bet updated immediately after you sync new people to your Ancestry tree.
A work-around to this is after you sync in a lot of new people from FTM, go to your web browser - log out of Ancestry - clear the cookies in your browser - then log back in.
I tend to do all of my adding of people & facts only in Ancestry, then sync those changes down to FTM. By doing this, it's not necessary to go through those steps. But if you prefer your workflow, this technique will take care of the issue you're seeing.