Shadows (pro) disappear on Unity restart

Version: Unity 3D Pro (3.3.0f4)

Frequently I'll find that when I restart Unity3D a prefab that previously created shadows on my ground plane will no longer do so. As far as I can tell, nothing has changed. If I create a new prefab with the exact same components and settings as the old prefab it will create shadows just fine. Eventually though the new prefab stops creating shadows as well. I'm not exactly sure what I am doing wrong.

The only shadows in my scene are those generated by a directional light.