iptables only NATing the first udp packet in a "connection"

Hi

boxes A and B are negtiating a very simple udp conversation:

A says "Hi" to B
B says "Hello" to A
and A replies with a message say "MMM"

this works fine
but now I want to "hide" box A

so I use box C

A says "Hi to B but B sees "Hi" from C
B says "Hello" to C which redirects the packet to A
A says "MMM" to B but B sees it from A

one rule on A:
iptables -t nat -A POSTROUTING -p udp --sport 5555 -j SNAT --to
"address_of_C"

and one rule on C:
iptables -t nat -A PREROUTING -p udp --destination C --dport 5555 -j
DNAT --to A


sniffing on B
I see
C --> B
B --> C
A --> B //arrrrrrr

on box A I see only one packet is matched


Thanks a head for any help