things to check???
Have you checked the blue water delivery tube to the upper washarm? They often rotted away and developed holes in them which would let the water not reach the arm or cut down the volume.You should check the check valve for an obstruction as Nathan suggested. The check valve threads apart for inspection /repair.Then remove the wash arm support and check to make sure that nothing is blocking the flow down thru the pump that supplies the wash tube to the top arm.
And needless to say, is the machine getting a full charge of water when it fills up? It should be 1/4-1/2 inch below the wash arm or just up to the bottom of the wash arm hub, or in your case, it should be nearly up to the top of the fine mesh filter where it would meet the wash arm. Just a hair below the space bewteen the wash arm and the filter.
Let us know!