Problem with SNAT

[Bgs]

  Greetings,

I have problems with the following setup:

A linux with two NICs. One with IP of 10.0.2.2 and one with 10.0.3.57.
I have DNAT-ed traffic coming in on the 10.0.2.2 that was originally 
sent to 10.0.2.1. (Another node doing the DNAT). I have problems on the 
route back so I decided to SNAT the backward udp traffic to  soource 
10.0.2.1 and send the SNATed packets back on another route.

I added the line to nat postrouting (-d target_net -s 10.0.2.2 -p udp -j 
SNAT --to-source 10.0.2.1) but the packages don't even seem to hit the 
nat postrouting chain. Let alone my SNAT rule.

Any ideas what could be wrong ?

Thanks
Bgs