While frozen yogurt is something you want to eat as a healthy option, it really works best with full fat yogurt. Also, the more protein the better. This recipe uses full fat Greek yogurt, which results in a super creamy texture.
If you use low-fat yogurt it has a much higher water content which means it will freeze into a rock-solid mass. You could leave it out of the freezer for longer before eating, but you'll find the melted parts are very watery and it will be very crystalised in texture.