There were two marriages for a James in Amherst Co., VA. Their last names were spelled differently, however. James Chastain married Phoebe Padgett on April 18, 1782. I have seen a message posted by Paul Rekow showing your Robert Chasteen (son of James and Phoebe) b. after 1774 VA. If Robert was a son of James and Phoebe Padgett Chastain, then Robert was born after they married in 1782. Let me know if I have misinterpreted Paul Rekow's chart.
The other Amherst Co., VA marriage for a James was James Shatteen/Shasteen to Nancy Kennedy on Nov. 8, 1788. This James served in the Revolutionary War and drew a pension. After his death his widow applied for a pension in Rockcastle Co., KY. The pension record tells a lot about them and their children. It does not mention a son Robert.
The Jesse Shatteen/Shasteen, Sr. I research has long been believed to be a brother of the James Shatteen/Shasteen who married Nancy Kennedy. No one has been able, however, to find documentation of Jesse and James' parents.