That depends on the feeling you want to give to your game, if you want to insist hardly on the "guess what combination is working", you can randomize as often as you want. On the other hand, if it's more like a puzzle game approach, with like, 1/2 combinations possible but only one that allows you to finish the level (the way I thought the game was at first look), you can randomize betwen levels, but resetting after each try seems a bit too much (to me). I hope it helps :)