It appears that the file names must be exactly the same in both places. When you do a backup, the backup date is appended to the file name and so Ancestry does not recognize it as the same file. Is there a way to inhibit the addition of the new backup date to the file name? I can tell that is the most current backup file by the file size and the file saved date in the finder.
Trying to edit out the new backup date to make it exactly alike to the Ancestry file name appears to work, but I wind up with duplicate family files on Ancestry.