Occlusion Culling issue (works like Frustum Culling)

Occlusion culling can only calculate occlusion from things that are present in the scene at the time you bake it. If the only thing present in the scene initially is the floor plane, then that's the only thing that will be an occluder. The fact that your prefabs are marked as static doesn't matter - they have to actually be in the scene to count for anything.

If your procedural level is always going to be generated according to a regular grid or something, then what you could try doing - I've not tested this, so it might not work - would be to place an OcclusionPortal on every grid square (possibly a pair of them forming a cross over the square), and baking them. Then, at runtime, when a particular square has a wall in it, you set the occlusion portals to 'blocked', and when a square is empty you set the portals to 'open'.