1) Check water level: The water level should be high enough in the base to touch the float. If too little water, there isn't enough to circulate with the proper amount of force.
2) Is there a "tower" that pops up from the base of the lower spray arm? While dishwasher is running, open the door and align the spray arms so they are positioned in the same direction (ie: toward you, or toward the side walls). Close door, allow to run 15 seconds and quickly open the door. Have the arms moved? Is the tower tipped slightly sideways? Check the base of the lower arm assembly. Depending on the model/brand, there might be one or two tabs that hold the arm/tower in place. If one or both tabs are broken, the assembly will tip slightly - just enough to redirect the force of water. The result - poor cleaning, esp the top rack. Solution: Replace lower arm assembly.
Hope this helps. Carole (footloose1)