it is probably because the mesh of the chair or the mesh of the robe is too low, one is slipping through the other because it is not detecting collisions properly.

My thoughs would be that it is the chair that is too low.  It you can put a high res cube in where the chair seat is, and parent it to the chair (so they move together) you should be able to get the robe to collide with the cube.

Just make the cube invisible when rendering.

John.

Fugazi (without the aid of a safety net)

https://www.facebook.com/Fugazi3D