To check, log onto your Altiris server and go to your Start Menu, All Programs, Altiris, Deployment Solution, and then Configuration.
Click Options, then head to your Transport Tab. Is Disable Multicast Checkmarked? If so, uncheck it. Also, go to your Disk Imaging Tab and check to make sure that "Use disk image multicast threhold" is set to a value not too high to test with. For example, for the number 3, we would have 3 clients connect up to each master.