This is where the tweaking of the physics settings comes into play. I suspect you already increased the Geometry fidelity settings mentioned earlier.

There are two things to try here in the following order:

1. subdivide/add more polygons to the plane  that is the floor - increasing this by a factor of 2 or four would be a good start. The additional Polygons and vertexes help the Bullet physics engine figure out what to do. (if you are using an older version of Carrara - v7 or earlier - then the physics will be more difficult to tweak since the Bullet library  <Carrara v8+>
   is much, much better than Carrara's original implementation)

2. Incrementally increase the collision fidelity above 100% - this will begin to significantly increase the calculation time.