Last Response showing as Never

I installed IP Address Tracker today. Once I added my subnet, I noticed that I had more IP addresses showing as available then there should be. Also, the ones showing as available show a Last Response of Never. A coworker of mine is running the same version of IP Address Tracker, but he is seeing an accurate representation of the subnet with only a fraction of the IPs showing as available. Is there something I am forgetting to do?