Remove the spray arms.
Soak them overnight in a tub of CLR.
With the spray arms removed,run the dishwasher so that water circulates without being restricted through spray arms.
This may dislodge some crud that can be wiped out.
Re-attach the arms and start the unit.
Once it fills and starts washing add 1 quart of white venegar to the water and let it compleete the cycle.
Glisten dishwasher cleaner and scale remover ==> http://www.repairclinic.com/PartDetail/Dishwasher-Cleaner/5304482929/1915432