PS3 controller only works in game on slot1?

My kids have bn playing minecraft multiplayer for ps3 fine no probs they just hit the start button to join the game n play together
but since yesterday only the controller that's assigned to slot 1 on the ps works 'in game'
The controller works fine until it gets to the game screen then it's unresponsive unless I hit the home/ps button if I reassign the controller to slot 1 it will then work but then the other one won't work in game

You cna only use slot 2 if you are the second player and its already in a two player game, you can't start a game with controller 2 and you can't play 3 player minecraft on ps3 either, its one or two players only

This may go without saying, but you can only play 2 player splitscreen with an hdmi cord and hdtv