Flexconnect mode AP can't join Multicast group address configured at WLC
Flexconnect mode AP can't join Multicast group address configured at WLC -> AP mode Multicast to Multicast. Since it is not joined to the Multicast group, it can't receive Multicast packet that are sent by WLC(and Multicast packet sent by flex central switching will be received by local mode APs only). If Multicast needs to be forwarded for flexconnect central switching then configure AP mode Multicast to Unicast. Remember, it is global config that's applicable for local mode AP also.
WLC 5500/WiSM2/8500 supports both Multicast and Unicast mode
When mix of local mode AP and flex central switching is used where flex centrally switched wlan need to forward/receive Multicast traffic then need to set "AP Multicast mode to Unicast only". (With Multicast mode enabled, WLC can't send one copy to the Multicast address and individual copies to the Flex mode APs.)
WLC 2500 supports Multicast to Multicast only
Flex central switched APs can't receive Multicast packets since it doesn't support Unicast mode. Use Local mode AP or Flex local switching as workaround in this case.