Some switches have bugs in the IGMP snooping implementation. This can cause IP multicast packets to get dropped. Usually an update to the firmware fixes the issue. In the worst case, IGMP snooping has to be disabled.

Running on virtualized systems oftentimes also causes IP multicast packets to be dropped, e.g. see [2]

See [1] for a description of IGMP snooping.

[1] http://tools.ietf.org/html/rfc4541

[2] https://bugzilla.redhat.com/show_bug.cgi?id=880035