Grave markers, tombstones, and other memorials erected in cemeteries can be a great source of genealogical information. The problem is that not everyone buried in a cemetery necessarily has a physical marker for their grave.
Some stones fall down and, like the remains of the person they memorialized, end up buried where they cannot be seen. Other stones disintegrate, break, or eventually weather away–perhaps even being destroyed by someone when they are viewed as being beyond repair.
Other individuals never had a marker. There might not have been the money to pay for anything besides a burial and the grave remained unmarked. If the deceased individual left behind no local family members, a stone may never have been erected or maintained adequately.
The genealogist is more likely to find a stone for an ancestor if that person died with adequate financial resources and family in the immediate area. Not every grave was marked with a marker and not all markers remain extant. If you think a relative is buried in a certain cemetery, see if that cemetery has records. See if the obituary mentions a place of burial. Determine if death records exist.
Other family records may indicate where the burial is–even if there’s no marker left behind.