Had this problem with a Whirlpool dishwasher a few months ago. My problem was that the top rack had slipped down a bit and the spray arm was unable to rotate freely - it was getting stuck. It is essential that this can rotate freely it, it has jets on the end that spray water.
I would also recommend putting in large plates with the bottom of the plate facing you as the plates tilt downwards and leave plenty of clearance to the spray arm. I always check that this spray arm can rotate freely before I switch it on. Normally use economy wash all the time, mind you I have a good habit of rinsing off excess dirt before I put something in the washer.
hope this helps