It helps if you think of the wheels applying forces perpendicular to the axis of the rollers. when the rollers are arranged as you had them, the diagonals were applying forces in the same directions, which led to problems. You can either arrange the roller axis as an X across the bot, or an 'O' around the bot... not quite sure which one is better, as flipping the bot upside down will result in the other configuration.
Good luck
