Sometimes there simply just isn't a marker at a cemetery for an ancestor. Whether due to ravages of time, weather or vandalism, today it is possible that a marker from a few generations back might not be there anymore. Religious beliefs may have precluded any time of marker, or the family may not have been able to afford to place one on the final resting place.
Recently, I had requested two photos of memorials at the FindAGrave website for my maternal 6th-great grandparents, Jacob SWAFFORD (1762-1845) and Sarah (LARKIN) SWAFFORD (c1772-1852). Both are listed as being buried at the Brushy Cemetery in Bledsoe County, Tennessee. There are several members of the SWAFFORD family with tombstone photographs in this cemetery.
However, a transcription of the burials at the cemetery at USGenWeb does not list Jacob or Sarah. Other SWAFFORDS, however, are listed.
It is possible that whomever created the memorial in the database was in error, and they are simply buried elsewhere.