SNAT will not be perform

You could also do a tcpdump on the client side to see if it is communicating with the VIP, or to James' point, directly to the server. If you are capturing the client side and server side and indeed confirm that source is unaltered, I've never seen SNAT just not work, so I would step through the config again. Is the SNAT enabled on the right VS in the right partition and route domain? Do you see the connection counter increment on the VS you expect and see the connection in the conn table? I also like troubleshooting with iRule. Setup a rule to log the connection to see where the breakdown is.