How can I fix my Diplomat ADP 8332?

My dishes were not coming out clean. On opening the washer during its cycle I found it had hot water in it but it was not circulating through the upper bldes; just the very bottom one that sprays upwards. After many attempts I gave up and stopped using the washer for six months. On a whim I tried the washer again and it worked o/k. After a month I'm back to the same problem; no water up top and dirty dishes. The washer drains out in its own waste and there does not appear to be restriction on the water supply. The sieve is totally clean; I always prerinse before loading the machine. Could this be a pump problem?