It just takes awhile sometimes, the system tries to find the perfect match and it basically happens to everyone every once in awhile. A lot of times it's because you are in a party with a lot of mixed levels, this usually is the case in ranked games but since you're in social slayer it doesn't make as much of a difference. IT also could be something to do with the spotty Live service over the last week, basically a good amount of people cannot connect to Live so possibly there is a shortage of players playing Halo/maybe they are getting disconnected (the latter happens to me a lot).