If the subtractor inside the bool has been polygonalized (hit the C key to make it editable) then it should be able to bool properly – make sure your headstone is also collapsed to polys as well. (I mean this shouldn’t be necessary but when the bool has a heart attack sometimes its neccessary).
If you are using R21 (and maybe R20) this is sooo much easier with the new bool tools.
If that still doesn’t do the trick make sure to subdivide the shapes being booled – sometimes there just isn’t enough geometry to give the bool a clue as to where to cut.
Also try the bool with High quality turned on or off and the same with the Create Single object. You won’t always get the best bool but the bool will often work in one of these modes and not the other.
You could also try using straight text, do the bool, make the resulting object editable and then bevel the edges of the text to get that chiseled look.