When you spawn a prefab with Gena it will be placed at some offset from the collision location (relative to the terrain or mesh that was collided with) - and that offset is based on the min and max position offset.
These are implemented as a range so that you can configure them so that no two spawns are the same if the values are different, or always at the same offset if they are the same.
When you drag your game object or prefab into GeNa, by default GeNa will pick up the height of the object in your scene if you drag it from your scene, or conversely will pick it up out of your prefab if you drag a prefab directly onto a GeNa spawner.
So if you prefabs locations are not zeroed out, or you are not paying attention to this then you will get unexpected results when you attempt to spawn your objects.
So check the check the Min and Max Y offset.
- < 0 embedded into terrain / collision object
- == 0 on the terrain / collision object
- > 0 above the terrain / collision object
If you are making POI (collections of objects to be spawned), then check out this video: