All wheel drive is permanent on that car, it doesnt just come on when needed. if you were drving in deep unplowed snow, its possible you actually lifted the car up enough off its wheels where it couldnt get traction. Believe me, I've done it before
When a 4-matic vehicle is off the ground (say on a lift) and in drive, the rear wheels will spin while the front do not as this is the path of least resistance. You can actually stop the rear wheels from spinning and the fronts will start.