I was shown this by some friends so don't give me credit for this.
But once you start searching and you have a hard time finding a game, try dash boarding, go to the big screen where it shows the master chief game, and press start, it'll pop up options for it and go down to where it says 'Quit'.
Then that'll restart the game, now search again and that'll help, it helped me out a lot!
Ps: if you're in a party with friends, go to party, then it'll say leave at the bottom, click that and it'll say multiplayer, leave it (it won't leave the party or game you're in) then do as I shown above, quit game. It'll retart and that should help joining friends and finding a game if it's stuck searching for one. If it doesn't then repeat the process. I know it's overwhelming but it helps, and it's what you gotta do for the time being.
Hope it helps!
llshinyll