In addition to the above, you could also let the machine run (empty so you have a clear view)Let the machine go through one of its complete fill phases. when you open the door, notice the position of the 4 arms on the hydro-sweep at the bottom. Also take note that the holes in each of those arms are arranged slightly differently. Once you made a good note, turm the machine on for about 15 seconds and open it again and see if the wash arm has rotated and the 4 arms are in a different location as well as if each particular arm has moved to a different area of the circular movement.
If the 4-way hydrosweep is not rotating, the most probably cause is the wash-arm support if this part has never been changed on this unit. and that part is no longer available, but a repair place may have some stockpiled (other members can direct you with that).