Most dishwashers have a screen and a mini garbage disposal and after time gunk builds up. By taking a cup of baking soda and adding enough water to make it pourable into the lowest part of the drain area and then adding 2 cups of white vinegar letting it set for 15 minutes following 4 cups of boiling hot water will clean most drains and removed the build up.
I had a problem with mine draining completely and when I took the screen out you would be amazed how much gunk was in there from years of use. It is recommended to do this at least once a year.