cant remove user body


im using a full body tracker and trying to put my 3D figure in top of the users biody, and this working. Then when i trying with puting a body eraser in then i got problems, i follow the tutorial, but either my figure disaper , or there is no reaction. I have used both a orthographic camera, and a real camera, and i have put this camera on top - but nothing is working. What can i do?