Fred, if you haven't done so already, you should check out ACME Brick's free masonry designer program. It has dozens of brick, cmu, and mortars you can combine in endless varieties.
How do you perform the draping operation? Do you drape the whole geometry onto the terrain in one (maybe as grouped)? It is most probably layered (from the imported CAD drawing), could you try to drape the geometry layer by layer? To me it seems that the drape tool somehow has its limits at certain cases.
