Tony,
Denise 'Fish' Fishburne, an extremely knowledgeable lady working in Cisco Proof of Concept Labs, has suggested this basic set of questions to answer on a particular router whenever there is a problem with the multicast:
- Who is the root of the multicast distribution tree?
- Where is this root?
- Who is the PIM RPF neighbor toward this root?
Very often, the problem can be isolated by asking yourself these questions on each router, going backwards from the receiver toward the source of the multicast transmission.
Unfortunately, without knowing more about your network, it is kind of difficult to suggest anything more precise.
Best regards,
Peter