Problems Multicasting

I am having problems getting all my PC's in a multicast session to image.
I set up a group of 8 PC's after I enter the session name for the last PC
it's display will show the screen just before the progress bar displays,
this will also happen if I try to image anything over 3 PC's.
I am running Zen 7 on the PC's Boot CD and server eDirectory 8.7.3.7. They
are connected through a Cisco switch
Any help would be greatly appreciated.