SNAT List (outbound SNAT) address translation problem

OK. Look like I have it figured out. Seems like routing related issue. After examining routing paths this morning I noticed that traffic destined for new network was traversing the firewall while traffic destined for old network was send directly to Big-IP interface. Still not sure if firewall was altering my packets or it relates to forward and return traffic taking different paths. My understanding is that SNAT listener alters source IP only for outbound connections originated from the hosts on the network behind Big-IP as they traverse through the gateway. However when host behind Big-IP sends a reply to inbound connection the listener on the gateway knows that and preserves the original IP. I could be off in my theory, but after fixing the routing everything is working as expected.