Particles stack

Hi guys,

I have a problem with particles. I created a particle system that falls to the ground, that system does have a WorldCollider, so particles collide with the ground. Everything looks fine :

But when i look from a different angle, particles stack together, and there seems to be an empty space between ground and particles, and i don't understand why.

My plane is just a regular box collider, and the particle uses horieontal billboard stretching mode. I used to use sorted billboard and have better rendering, but unfortunately it's not working on IOS.

Does anyone have an idea how to fix this ? It's pretty urgent...

Thanks a lot.