Why won't my dishwasher clean now?

I have a Whirlpool model GSD4030 dishwasher (6 years old) that has suddenly lost cleaning and rinsing ability. Currently it appears that something is stopped up, but I've cleaned the spraying surfaces as good as I can with little notable change. All cycles appear to be functioning properly; but the end of cycle affect is soapy, still uncleaned dishes. Can anyone suggest where to look and what to do about this one?