One tip (related to Branr's) is to level the ground that the excavation site is on. If the ground that the spawner is on is completely level, it will usually spawn but it might take a few game days. I don't level the whole lot (unless it's not built on yet), just the area that the spawner is on.
One of the easiest ways to do this is to put the spawners on vacation home lots or, e.g., the Landgraab estate in France. Excavation sites not spawning on uneven or sloped ground seems to be a common problem since Ambitions. BTW, Saltypaws, I could not excavate on the burial grounds until I leveled the ground. I guess where you can excavate must vary. But I can't excavate anywhere unless the ground that the spawner is on (and generally the whole area that the green square is when you place the spawner, although IIRC sometimes it's ok if the corners are not completely level) is completely level.
Does anyone know how many of which level of spawner is in the various vacation countries? I usually add spawners on level ground to my vacation areas. However, I'd like to have approximately the same number of low, medium, high, fragment, general (etc.?) sites as an unmodified game would have if it were working properly.