Particle geometry not self shadowing

[RossOleary]

I'm guessing this is something I'm doing wrong but my particles are not casting shadows on each other. On the right is a sphere on the left a sphere packed with sphere particles. 

[RossOleary]

I'm guessing this is something I'm doing wrong but my particles are not casting shadows on each other. On the right is a sphere on the left a sphere packed with sphere particles. 

I seem to have fixed it.