You want to use debug functions to isolate problem with multicast forwarding.
Cisco routers have several useful debug features that you can use to isolate multicast problems. The first is a general command that shows how the router maintains its multicast routing tables when it hears from sources and group members:
debug ip mrouting
You can watch the actual multicast packets for a particular group using the command:
debug ip mpacket
And the other commonly useful multicast debug command looks specifically at IGMP information:
debug ip igmp
As with all debugging commands, you need to be extremely careful because sometimes the sheer volume of the output can overwhelm the router. So it is usually wise to try these commands one at a time, and disable all debugging with the command undebug all before trying the next command.
The first debug command, debug ip mrouting, shows how the router creates, updates, and deletes multicast routing information:
debug ip mroutingIP multicast routing debugging is on Router# 17:20:27: MRT: Create (192.168.5.1/32, 184.108.40.206), RPF Ethernet0/0.0.0.0, PC 0x33A89D8 17:20:43: MRT: Update (*, 220.127.116.11), RPF Null, PC 0x339F96C 17:20:49: MRT: Delete (192.168.3.2/32, 18.104.22.168), PC 0x33AB26A 17:21:43: MRT: Update (*, 22.214.171.124), RPF Null, PC 0x339F96C 17:21:49: MRT: Create (192.168.3.2/32, 126.96.36.199), RPF Ethernet0/192.168.5.1, PC 0x33A89D8 17:22:13: ...