My IIS 7.5 Log doesn't show visitor IPs some times. Do you know why?
It could be a result of application request routing in iis.
Take a look at this article for a possible resolution:
You may also be able to add the REMOTE_ADDR into server variables under URL rewrite. This can help if you are using multiple servers (in which case you need to add the remote_addr to each server).
Lastly, also remember that proxy hosts wont return the end client but just the proxy's ip.