Split spray arm: The spray arms could be clogged with food particles or mineral deposits. You can try and fix it with an object such as a pipe cleaner, straight pin, or toothpick to clean out the holes or you can just replace the spray arm.
Filter: Some dishwashers have filters that clean themselves; however, others have filters at the bottom of the tub that you have to clean. These types of filters aren't very difficult to replace.
Pressure switch: A bad pressure switch can sometimes prevent the dishwasher from filling up enough. and can lead to extra water leftover after the cycle. In this case you would have to replace the pressure switch.
Positioning your dishes: Depending on the way that you position your dishes, some will get cleaned more than others. Put big bowls and such on top so they don't block the water from the bottom. Obviously, stacking a dishwasher is pretty basic, but there is some kind of strategy to doing it apparently that can make a big enough difference in how well your dishes get cleaned!
Water temperature: The water that goes into your washer should be at least 120F and at most 150F. If your dishwasher is hooked up to your sink, turn on the sink to hot water for just a couple seconds when you start your dishwasher to make sure that the water going into it is already hot. That can make a big difference
My problem ended up being with my spray arm. I didn't want to go through the do-it-yourself hassle of cleaning it out and everything so I just bought a new one since there's a chance the one I have now would just clog again. Applianceparts.com also sold the spray arm I needed. They had next day ground shipping so I just went ahead and bought the part online after I figured out the part number and everything for the spray arm.
Good luck repairing!