What NAT type does your XBox report when using the built-in XBox network tests? For that matter, have you ran any of the XBox network tests, and if you had or do, you might post the results here. Empty lobbies and unable to matchmake with people is almost always one of two things. The most common is having a strict NAT. The second most common reason is simply having a poor quality connection that is dropping packets or has a lot of long ping times. Most of the XBox built-in network tests will tell you what your NAT is as well as the performance of your connection.