Was the soldier buried there during the war or was he buried there in the post-war years?

It is likely the grave sunk due to the coffin collapsing or the grave was robbed. It is also likely he may never had a stone or the marker (stone, stack of rocks, wooden slab) did not make it over time. Both of these are not unheard of or unlikely. If the grave was moved from a private cemetery, I do not believe the family that possessed the plot would have allowed the ground not to be repaired for future use. It is very unfortunate but the landscape is replete with cemeteries that have been abandoned.

What is often the case is the families emigrated to other parts of the country leaving behind the remains of relatives and ancestors. Many, many Georgians left and went to Texas, Arkansas, and the Indian Territory (present day Oklahoma). Look at the names of the counties and towns around Dallas and Fort Worth area and you will see many names not unfamiliar to Georgians.

Respectfully,

Gerald D. Hodge, Jr.
War Between the States Historian
Historian: 39th Georgia Volunteer Infantry Regiment
http://39thgavolinfrgt.homestead.com/39thHomepage.html