Did you install the updated mod?
If so, are you sure it doesn't capture the IPs for everyone?
You know it only does when people log in & register.
Some people rely on cookies and don't login for even weeks... thus, the IPs of those won't be recorded until they login the next time after logging out/cookie expire.
I'm running it on a board with 3000+ members, and it's working fine so far... except, it doesn't record the IP of those who are already logged in.