Why Can't I join any party on Black Ops, and Why Can't my friends join my party on Black Ops?

Whenever I try to join a Black Ops party, It doesn't load, Or it will go to the Multi player title screen saying, "The Game Session Is No Longer Availlable" , My nate is Moderate, and all Party Privacy are on Open, The person I cannot join (or Vise Versa) NAT Is Open, Why is this happeneing? I can not also join some other friends groups also,