It's not because the dishwasher is cheap, decrepit and was installed during the Harding administration. No, your dishwasher doesn't clean dishes because you already did!
So says cleaning expert Arianne Cohen, who cites pre-rinsing dishes as the number-one fault people make when using the machine. In addition to wasting water and energy, it has the opposite effect on the dishes. Cohen cites a dishwashing engineer: "Dishwasher detergent aggressively goes after food ... and if you don't have food soil in the unit, it attacks the glasses,
and they get cloudy." That leads to the process known as "permanent-etching," something KC Lunch Spots observed on the glasses at a local diner.
Since detergent attacks food and not dishes you should add detergent based on "food soil" not how full the dishwasher is. Other hints: Despite all the fancy gizmos companies come out with, powder detergent is best. Never place bowls upright, unless you like harboring an environment for bacteria. Same goes with cups. The water doesn't drain and dirty water sticks around.
Finally if you're already doing all of the above then yes, it may actually be your dishwasher causing the problems. But before buying a new one (or heaven forbid, doing the dishes in the sink) make sure to examine the filter and spray arm. Often times they are clogged with debris and just need a good cleaning, something that can be done in the sink since both parts are detachable.
(Image via Flickr: Wade Rockett)