Okay, I've noticed something about the servers. Ranked servers anyway.
Everytime single time I was able to get into an SCEA ranked server, the match was just starting. Scores would be in the 0-5 range. This means that the error message of "Game is full" is incorrect, and it should be saying "Game has already started".
So, when you want to join a server, look at the info box next to it. You should locate the number that says Time Remaining and see if it matches the time of the match. They are right on top of eachother so it should be easy to spot two identical numbers. If they match and it happens to be the type of game you like, join FAST because 1) This is the only time people can (supposedly, what I think) join the game and 2) Even though it says 20/20 minutes remaining, you have to join within the first half of the minute to not get rejected. At least this was what I have experienced. Hope this helps you join your games.
I guess they did this to prevent people from joining mid or late game where they don't get a full chance at a good score. Though I don't really care, I just want to be able to play within 3 minutes of starting up the game. =/