#show ip mroute 188.8.131.52
(10.1.12.4, 184.108.40.206), 00:00:01/00:02:58, flags:
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Ethernet0/0, Forward/Dense, 00:00:02/00:00:00
Vlan111, Forward/Dense, 00:00:02/00:00:00
Take note of above output, if you see that the incoming interface is null and the neigbour is empty on a router (bold red), it usually means a missing "ip pim" on some interfaces or an issue on the underlying unicast routing issue. If it's latter, perform standard routing troubleshooting on the underlying IGP or static routes.
There is this good blog post that further elaborates the detailed steps on RPF troubleshooting.