Nvidia Flex - bullet solver - how to keep elements inside the box

Hi,
I am doing this Bullet solver tutorial - TouchDesigner - Bullet Solver, Flex Solver COMP - Interaction Tutorial - Basic Setup - YouTube - however when I spin the main Actor box that contains the particles and small boxes, the small boxes inside the main one keep falling out.
Is there a way to make sure the elements stay inside the Actor (in the tutorial called comp_for_flex)?
Thank you very much for your help
Pauline