Dishwasher not cleaning properly

a low water level will certainly cause your problem. the water valve may indeed be bad. first, check the water level mid-cycle. let the machine really get going. then crack the door. let the water stop flying around. and open it up to check the water level inside the machine. your heating element should be partially or even completely submerged.
if the level is high enough, your water valve is most likely fine. and i have no idea what to do.
if the level is low, the valve may need replaced. another common reason for this is low water pressure in the entire house. a new valve will not help you (or me). if you are handy, you can pull the 'restrictor plate' out of the inlet valve. it is under the screen filter where the water line hooks up. it looks like a washer with a small hole in the middle for the water to flow through. it should pop right out for you. this will allow the water to flow in more freely. don't forget to put the screen filter back in.
also, that screen being clogged may cause your problem. good luck