Dishwasher- water flow seems OK, upper arm not rotating?

I have a Kenmore dishwasher Model # 587-15152400, about 2 years old. The water spray seem to come on because I can hear water splashing when it comes on but the upper spray arm does not rotate . I can tell whether it rotates because I marked the arm and it did not show any movement.I took the whole assembly off the washer including the funnel and using a garden hose at full flow and pressue ran water into the funnel. This showed no rotation of the spray arm. The arm seems free to rotate when I spin it by hand. Is the problem a matter of not enough water pressure, too much friction of the rotating parts or loss of water due to loose rotating parts. Any body have any ideas as to what is the cause of this problem Thanks for any help.

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