[c-nsp] Cannot join a few multicast groups

[ML]

I'm having trouble joining some multicast streams.  The upstream router
joins it fine.  The upstream has (*,G) and (S,G) in the mroute table.
Downstream doesn't have (S,G).  This is a sparse mode environment with a
static RP.

>From the router with trouble I can ping the mcast group and get
responses from the group members.  I can ping the unicast source of the
group, the RP..anything I can think of to rule out an RPF problem.

Keep in mind that we're having trouble with 5 out of ~400 multicast
groups.  Of course nothing that we know has changed at all.

Thanks