I solve it already... but yes, this was exactly what i did, the problem was that the interfases were configured in different zones and when it was trying to return the package back i received a "zone missmatch error(i saw it in the a flowtrace file". This is something that doesn't happen on the SSG (almost sure).
my flowtrace file:
Dec 15 18:46:13 18:46:12.987602:CID-1:RT: route lookup: dest-ip orig ifp reth2.0 output_ifp reth1.0 orig-zone 10 out-zone 9 vsd 2
Dec 15 18:46:13 18:46:12.987602:CID-1:RT:
Reject route in make_nsp_ready_no_resolve. zone mismatch
The traffic was not returning through the incoming interface.
resource: http://kb.juniper.net/index?page=content&id=KB15545&smlogin=true
Regards,
Layard