Hi Gyus..
I have very simple topolgy,

R1 is the Hub, R2 nad R3 are spokes,
R2 joined to 224.10.10.10 on int loo 0
R3 will ping 224.10.10.10 from its ser int
all interfaces configured with sparse-dense mode.
NBMA mode is enbled on the hub router although that the frame-relay supports Broadcast.
eigrp is the IGP between all links,
the fast ethernet int is currently disabled, i put it just to help me figuring out the issue, when I used it instead of serial, multicast traffic between R2 and R3 worked,
RP is static and installed on R1 loo0 interface
the problem is that when I pinged group 224.10.10.10 from R3 serial interface or even loop interface I got reply only for the first rping, then all ICMPs timed out. what happend is that R1 sends directly prune meassge to the source instantly after adding (S,G).
here is the PIM debug output from R1 : as it shows it is not RPF issue:
*Mar 1 01:24:52.643: PIM(0): Received v2 Join/Prune on Serial0/0 from 192.168.1.2, to us
*Mar 1 01:24:52.647: PIM(0): Join-list: (*, 224.10.10.10), RPT-bit set, WC-bit set, S-bit set
*Mar 1 01:24:52.655: PIM(0): Check RP 1.1.1.1 into the (*, 224.10.10.10) entry
*Mar 1 01:24:52.659: PIM(0): Add Serial0/0/192.168.1.2 to (*, 224.10.10.10), Forward state, by PIM *G Join
*Mar 1 01:24:52.663: PIM(0): Prune-list: (192.168.1.3/32, 224.10.10.10) RPT-bit set
R1(config-if)#
*Mar 1 01:24:59.359: PIM(0): Received v2 Join/Prune on Serial0/0 from 192.168.1.2, to us
*Mar 1 01:24:59.363: PIM(0): Join-list: (192.168.1.3/32, 224.10.10.10), RPT-bit set, S-bit set
R1(config-if)#
*Mar 1 01:25:37.431: PIM(0): Received v2 Register on Serial0/0 from 192.168.1.2
*Mar 1 01:25:37.435: PIM(0): Send v2 Register-Stop to 192.168.1.2 for 0.0.0.0, group 0.0.0.0
R1(config-if)#
*Mar 1 01:25:52.055: PIM(0): Received v2 Join/Prune on Serial0/0 from 192.168.1.2, to us
*Mar 1 01:25:52.059: PIM(0): Join-list: (*, 224.10.10.10), RPT-bit set, WC-bit set, S-bit set
*Mar 1 01:25:52.063: PIM(0): Update Serial0/0/192.168.1.2 to (*, 224.10.10.10), Forward state, by PIM *G Join
*Mar 1 01:25:52.403: PIM(0): Building Periodic (*,G) Join / (S,G,RP-bit) Prune message for 224.10.10.10