Water does not flow to upper rack water dispensors for cleaning.

All of the water coming to the upper spray arm comes through the feed tube. If the feed tube is blocked or the probe flapper is hung open, water will not flow properly to the upper spray arm. Be sure that when you push in the upper rack the manifold probe is in position in the feed tube. Try adding more water in the tub to see if the upper spray arm starts rotating. Other possibilities; low water charge, cracked spray arm & poor wash pump performance.

Make sure that that upper arm is free to turn. Check it while you have dished installed, because sometimes, dishes on the lower rack may stick up too high and hit it. Otherwise, check the back of the water-supply tube attached to that upper rack.... Is it lining up with the water feed port or hole at the backof the dishwasher? Is there a gasket that is either ripped or missing? And check that wash arm carefully.... Might it be split or bent? Are the holes on the bottom clear? (Those holes are usually angled to push the arm around; if they are clogged, the arm won't rotate.)

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does this have the water conduit tube going up the inside of the rear wall where you can see it,if so,the conduit "horn" where it connects to the tube that feeds water to the upper arm with repeatedly opening and closing this horn wears out and lets water get past here and lowers water pressure to the upper arm

Trouble shooting here... but the same pumb that circulates water... vents it.... So, if the vent valve is clogged with food debris, then you would not retain enough water to wash all the upper rack dishes...
I recommend inspecting the exhaust tube port of the pump (a selenoid is the operating device for that valve). Be sure that it is able to operate freely (close all the way. Otherwise, it is just the pump intake!! only choices...you have already eliminated the rest (GOOD JOB)

there is a chance that the main washer motor is not supplying enough water to the upper spary arm.the possibilty that there is not enough water in the bottom of the unit during the time it fills up would cause this problem check your water pressure.if this is good then the main wash motor may need to be change

I removed the lower spray arm, and then the filter. I found that there was a lot of buildup under the filter area. I scraped the buildup off. I found that it was restricting the input of water flow to the pump

Check the pump out. Even though the Hose is and tubes are clear it does not necessarily mean that the pump is not pumping water to the top sprayer. One way you can do this is take off the upper spray tube and lay it in a glass or bowl and turn on your machine for a minute or so to get the water flowing if it flows then I would take off the spray assembly itself and make sure that there is no blockage on the bearing race

the only way you can tell if the upper rack is not getting enough water is to turn a glass right side up in each corner. then run for 2 minutes and see if there is water in each cup.
if you still dont get water, then you have checked the pump, the only thing left is that you are not getting enough water into the unit.
start the d/w at a normal cycle, let it fill and go into the wash cycle, takes about 3 minutes, then check to see if the water level is just below the heating element. if so then it has to be the wash pump being plugged.
if not enough water then you should change the water inlet valve. some times they do get plugged but also make sure you are getting full water pressure to the unit as that could cause not enough water into unit. the faucet under sink could be at fault i have seen valve work but not let enough water in, so dont be surprised if you need a new valve.

check water level in machine
may be not filling enough and the wash pump is starved from water thus not pushing water to upper arm
drain water from machine dismantle and clean out air tube to pressure switch take off air tube at pressure switch and blow back into sump then pinch air tube with fingers to stop air from flowing back up tube then refit to p/s
run and see if that solves problem

You could try running it with the upper arm removed, just so you could be sure that the pressure is not getting to that point - unless you are absolutely sure the upper arm holes/channels are all clear then descale it overnight with some vinegar - just might clear something. if there is obviously only a little flow of water then perhaps you could try getting vinegar into the supply pipe using a basting bulb/syringe or similar - again leave overnight and you may get lucky. The next thing would be to take the machine apart so you could follow this hose through to the pump - undo it from the pump and blow through it or see if you can get a piece of wire in it from the pump end. try vinegar again. check the pump outlet for blockage. Hope this helps :)