Balls always collide with world boundary but inconsistently with other objects, why? (Arcade Physics)

[01AutoMonkey]

So I have a world, a box made out of 4 long and slim immovable arcade physics bodies, and about 40 balls within that box with velocity of 100 to 300, the framerate stays within 40-60 FPS.

 

The problem I have is that sometimes due to some factor balls just move right through the box, but strangely never through the world boundaries (or at least not to my knowledge).

 

Is this just a FPS problem? Do I need to maintain it at 55-60 or? Or could it be a bug in my code? What can I do to get consistent collision using Arcade Physics?

 

I execute the following on every this.update:

this.game.physics.arcade.collide(this.itemLayer, this.itemBoxLayer)

[Massemassimo]

Same problem here, have been looking for a solution for a few days now, but nothing helps. Really frustrating.