It might be that you have to up the number of subdivisions for the object.
I think it works by calculating the colides between the verts and the lines between them to make a colision plane, if I remember reading a post by Duncan Brinsmede (think thats his name, one of the principle scientists at autodesk) try smoothin your cube so it has more subdivisions and give that a go.
"No pressure, no diamonds" Thomas Carlyle