The upper spray arm may be blocked, which will prevent it rotating. To test it put an empty mug on the top basket and operate on wash for around 30 seconds, the mug should then be full of water.
If not, remove the upper spray arm and check inside the small holes in either end, these get blocked with food.
Just tapping the spray arm on a work surface you should be able to work the bits free out of the large hole in the centre of the arm.
Tony Cream
October 2005