If there is a Cisco device between the two units the Cisco may well have the default where IGMP snooping is enabled. When this option is enabled the Cisco device will not allow the Multicast traffic to pass between its interfaces to disable this option you can use the following commands:
No ip igmp snooping
On the link below is a Cisco KB that explains what the IGMP snooping is for and a couple of other options you can use to work around the IGMP snooping without having to disable it.
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008059a9df.shtml