Problems with lightmapping

I’ve used the method in this video to bake AO into my prefabs. I use ambient light as my light source with no other added sources. For reflections I put a plane underneath the hex tile so as to illuminate the base, however this doesn’t seem to be working.

The top of the hex tile is properly AO’d, but the base is just diffuse colour as you can see in the attached picture.

Also attached are the bake settings:

Does anyone have a solution to this? Any help is appreciated!

EDIT: This was baked with ambient intensity at 4.0, not at 0 as in the picture.

did you set the gameobject to be static?