If your supposedly clean dishes are coming out of the dishwasher with a scummy residue, then there are a couple of things you should consider:
1) Make sure your machine has plenty of rinse aid. Usually the intake for this helpful blue liquid is located in the door near the soap dispenser.
2) Check your owner’s manual in regards to the detergent you are using. Some dishwashers require the use of the powder detergent for best results. If you are using liquid dishwasher detergent and you are noticing residue (especially on your glassware in the upper rack), try changing your detergent to the powder version and see what the results are.
These are the two most common reasons for soapy residue on clean dishes.