LTM configuration for external api

Hi all

I need some help. I have configured one vip which pool member is a public ip . One of the internal server will communicate with this public ip. From f5 i can curl this ip which is pool member. But with vip i am not able to get the result. From this ip internal server will take some information and get back to the server again. What could be the possible reason , anyone can help me with suggestion?