Core Issue
When a router forwards a multicast packet from one interface to another, it decrements the Time To Live (TTL) value in the IP header by one. It sends the packet only if the resulting TTL value is not zero and is greater than the multicast TTL threshold value of the outgoing interface when configured. If a multicast application on the source allows the setting of a TTL value and results in not matching the mentioned criteria, the packets are dropped by the router.
Resolution
For information on diagnosing this problem as well as solutions, refer to these sections of IPMulticast Troubleshooting Guide:
Source and Destination Physical Connectivity
Source and destination exist on different subnetwork
