If you can spin the spray arm by hand then it's OK
The rinse water will have little bits of crud in it - food debris and bits of limescale - unless it drains off properly the drying process will bake it on hard.
Consider a dishwasher cleaning product.
Rinse off the worst of the food debris before loading the machine
Above all - use good quality liquid rinse aid - to let the water drain off properly.
Good luck...
M Berry
October 2014