Hi AlexC,
Thank you very much for the file and for using DropBox.
Some of these Spheres fall on the zero-dimension edge of the plane. My suggestion would be to create a box as below. Please note that the box does not need to be seen to work. So you can visually present the flat-based box.
I have set up different Sphere resolutions; please pick the one that feels best to you and delete the others.
Please check the Attribute Manager> Mode> Project> Simulation> Simulation parameters; I have increased those.
My best wishes for your project