Joystiq.com are reporting that they and other users are having problems with the new multiplayer maps that were recently released for Gears of War. They said that normally "tons of games" would be listed when searching for multiplayer games to play but that they were only finding one or two. The Gears of War official forums are cited to back up their claims.
They've e-mailed Epic Games for an answer and are currently waiting for a reply. Judging by some of the comments made by readers of the article one of the possible reasons for the problem could simply be that not that many people have bought the new maps and are waiting for them to become free.
Until an official response comes from Epic though, there's no hard evidence for what the source of the problem is.