I would recommend using
tcpdump
to look at the traffic flow:
and check /var/log/ltm to ensure that there are no errors. Since you have the http profile applied, the flow should look like this in a tcpdump:
CLIENT BIG-IP SERVER
--- SYN ---> |
<-- SYN/ACK --- |
--- ACK ---> |
--- ---> |
+ --- SYN --->
+ <--- SYN/ACK ---
+ --- ACK --->
+ --- --->
where REQHDR are the request headers. On the second half (between the BIG-IP and the SERVER), the source address will be the SNAT source address (172.16.254.50).
Having said all of that, the SNAT source address on the BIG-IP (172.16.254.50) must be able to reach the public addresses (since it's an RFC 1918 address, I assume it is subject to further address translation somewhere else?) and there must be a route back to that address.