Meshes penetrate through the Kangaroo Floor

Hi,

I set up a group of mesh and drop it down using the unary force. My original intention is let them stop when they touch a surface below, but this attempt failed. Then I turned the Floor on in the kangaroo settings and hope it could stop the meshes from further dropping, to my surprise, all the meshes continue to drop down without any halt at the height of 0.

I am not quite sure if there's something wrong in the formation of the meshes.My questions is how to control the meshes to let them stop on a specific surface?

I have attached the file below.

Thanks,

-Jerome