Cannot see traffic

Hello all I have an 8.2 Professional box that when I run tcpdump doesn't see any traffic that is sent to and from the localhost. For example tcpdump nmap localhost tcpdump doesn't see the traffic at all, but if I try to nmap a remote host it sees everything. Also if someone sends any traffic to me tcpdump sees it. I also tried ethereal and even snort to make sure that it was not a problem with tcpdump. Any help will be appreciated. Thx! Jose