Are you from North America?
If so, try playing the collection 4 and collection 1 playlists early in the mourning.
You do this because, most players of mw3 on pc, are from Europe. So, the time zone is different. Any other time of the day, play with DLC off.
This may sound stupid, but it might be reason why it's very hard to find a match.
I hoped this helped.
Also, not many people have bought the collections, so there might only be one game, and you won't be able to join it.