Particle System Batching

I am familiar with the requirements for dynamically batching objects in Unity. I am curious about the requirements for Particle System Batching. I am using Unity 5.3.5p2 currently. I created a simple test scene to experiment with batching. Unity batches the default particle system, but I cannot get Unity to batch any other particle systems. What are the requirements to get Unity to batch particle systems?

I remember reading about this - (guess from terrible memory) - do they have to be on the same z plane and be under a specific polygon count?

What do your other particle systems look like? Same material?

In the tests I did, every particle system was an exact duplicate of the same prefab.

Can you share the project? Do you have receive shadows enabled on the particle systems?

Yes, I can submit a copy of the project. Do you want me to open a support ticket and share the project? Or is there a different way I can share the project with you?

Open a ticket and post the bug number is easiest