Top Arm not spinning on Whirlpool DU1145

It looks like there is not enough water pressure to spin the spray arms. Loss of pressure may be due to various things, like damaged seal, cracked tubes, clogged filters, screens etc.
I suggest you take a look at the technician repair sheet that should be stored inside the front panel at the bottom of the door. Remove that panel under the door with screw driver. Inside, attached to the panel, you should find a troubleshooting guide and solution for technician. But it is easy to follow. Look at the section under "poor wash" and follow its recommendation. I had exact same problem - not enough water pressure. In my case, the chopper screen was clogged and the chopper assemply was broken. I followed the repair instruction above to remove the chopper assembly, cleaned the chopper screen and the washer now is as good as new. I will need to replace the chopper when the part arrives.
Good Luck
/Dara