Okay, I compared the traffic list with the client list. It is definitely a specific set of things that are missing.
1) Printers. I have three printers. One hardlined and two wireless. They are listed on client side, but do not show on traffic monitor. They are all Brother printers, but I doubt that matters in the solution.
2) Cameras. I have an IP110W camera wirelessly connected. It shows in client list and not on traffic monitor side.
3) Game adapters, TEW-647GA in this case. It acts as a remote ether bus (sort of like a switch). The devices behind it, I need to test still to see if they do show up. This one may or may not be an issue, since he has an IP, but really is kind of like another remote router.
I will try some more testing and see if I can pin down any more specifics.
The traffic monitor is only for traffic accessing the WAN, so it would make sense for printers or cameras not to show up.