Save a simulation and then add a new body into it

Hi all,
Firstly I build a simulation including a soil buffering and I save it in a .gz file.The critical code I use is :

if utils.unbalancedForce()<0.05:
  O.save('buffering.yade.gz')

Secondly I want to add a potential block into the simulation.In the second py file, the critical code I use is :

O.load('/home/liu/buffering.yade.gz')

But the visual window just shows the soil buffering and the potential blocked defined in the second py file is not displayed.
I checked the number of the bodies in the simulation.It truly includes the potential block ,however,which is not shown in the scene.
How to solve it?
Thanks in advance!