Light Mapping glitch on beast bake - big black splotches

So I'm baking in lightmaps for a scene, and two sets of geometry (which have been chopped into rough shards so as to be breakable) are coming out of Beast with what I assume is bad atlasing. The atlas images, themselves, look as I'd expect - I'm using dual lightmaps, and as you can see, most of the scene is working fine.

I have no idea what's causing these glitches though - can anyone offer guidance?

Thanks very much!