I have run into similar problems, with it flat out refusing to render shadowmapped shadows. In one instance, repeating an animation a second time produced shadows (no other changes). I just checked and the did an update to, I think 5.0.4.325. Maybe that will fix the issues... Reading another thread (other firefly problems), svdl suggested deleting all tmp files before doing a problem render (when all else fails, reboot the computer as well). It is worth a try...