I write a program: 
1. CComPtr iSwModel--> get the assebly document   "CComPtr iSwAssembly_Doc;" then get two components "CComPtr iSwOne,iSwTwo;"
  2. get motionstudy   for each timestep of motionstudy {            iSwModel rebuild;            Findmin (iSwOne.body.point, iSwTwo.body.point);            return List;  }   

To my surprise, the transformation matrices of iSwOne and iSwTwo are changing with time. However, the points on them are not changing positions. So the final return List is full of one value....  
Can anyone help me?
Thanks.