Baked scene lighting... everything looks fine except one mesh gets no AO?

So I have a relatively large outdoor scene, got everything baked decently for a test at least with 0.01 indirect res and 10 texels per unit. The only issue is theres one mesh, my ground plane, that is getting lightmapping (you can see the distant shadow in the screenshot below, they are lightmap shadows from the mixed lighting shadowmask), and you can clearly see all the nice AO playing on the rocks, but theres NO AO where the trees and rocks intersect the ground plane mesh. I can’t figure it out for the life of me.

Bump!