I learned something recently about tombstone generation. My sim had moved out of her childhood home, and then my sim's mother died, but there was no urn/tombstone left at her mother's lot. There was something about this particular lot that prevent the urn/tombstone from being generated.
The lot also had my sim's father's tombstone - he died while my sim was still living there. After some testing, I learned that because I'd used the "moveobjects" cheat to move some grass around her father's tombstone, that prevented her mother's tombstone from being generated.
From a previous save, I removed the grass and other objects around the other tombstone (my sims's father) and then when the mother died, the tombstone was generated properly - well, actually intersecting the father's tombstone. I guess the game wanted to place the tombstones near each other and couldn't because of my use of MOO.