Why can't I add a controller to my roster?

I've just bought the Halo Master Chief Collection, and I'm having certain issues trying to add another controller to the roster so I can play a co-op campaign. When I turn on the controller it appears that it's recognized by the roster and I can see an extra "press A to join" appear there, but when I actually press "A" on the second controller, it has no effect, and there's still only one player.

What's going on here?

Sometimes in a menu it will not let a second controller(or third or fourth) join, but for me, I just need to back out to the main screen, press start, and then add the controllers on the first screen, or the one that follows the main menu.