The Virtual Server cann't be reachable, but the members in the pool and the pool itself is fine. why ?

Do you have SNAT enabled on the Virtual Server?

 

 

It looks like a local area network routing triangle issue - wherein the client contacts the virtual server, the connection is then sent to a pool member, and the pool member directly responds to the client instead of routing the response back through the LTM. If SNAT is enabled, the forwarded connection to the pool member will inherit an LTM configured IP address, and as such responses will route back through the BIG-IP before reaching the client.