Original post by: mayer ,
Anything that blocks the spray arm can prevent dishes in the upper rack from getting clean. Most often this is a tall item in the bottom that stops the upper spray arm from spinning. If your dishwasher doesn't have a spray arm on the upper rack, something may be blocking the spray tower that is supposed to reach the upper rack. Too many things crammed into the top rack, bottom rack, or both can keep the top rack from getting clean. Water needs to be able to make its way up from the bottom, and too many dishes can prevent that from happening. Too many dishes in the top rack keep the water and detergent from moving around or making its way between the dishes. Minerals from hard water and residue from detergent can clog the holes in spray arms, preventing water from reaching the upper rack, even if it has its own arm. Even if the water is managing to spray out, clogs can limit the amount of water so there's not enough to do the job right. The rinse cycle may not be able to remove detergent residue, which can also leave the dishes looking dirty. Even if the dishwasher is not overloaded, if items on the top rack are placed improperly they may not be able to get clean. Cups and bowls, for example, should face down; otherwise water collects inside and the dishes don't get clean. The face of each dish should face toward the center of the dishwasher. Place glasses between spindles rather than on or over them so that water doesn't get trapped. Low water pressure can leave the top rack dirty. There may be just enough pressure for the spray arm on the bottom to operate normally, but not quite enough to reach the top. This can be caused by a kink in the dishwasher hose or low water pressure to your home. If it happens only occasionally, it may be caused by other appliances or activities using water, such as the clothes washer, bathtub and sprinklers.