Dishwasher not washing dishes properly

Thanks akubra_lands, it worked for me too on model DW28W. I thought I should elaborate on your answer for anyone else who comes across this while researching the same problem.
The repair is ridiculously simple (takes about 10min) and the only tools needed are 1 philips/posidrive screwdriver and 1 flat blade screwdriver. Assuming you've unplugged it from mains power:

1. Tip the dishwasher on its right hand side (looking from the front).
2. Remove 4 screws from the metal base - 2 front and 2 rear (the rear ones have anti-rattle washers on them).
3. Pull the base away, taking care of the pair of wires leading to the flood indicator float switch which is still attached to the base.
4. Remove all 4 screws which fasten the solenoid valve to the pipe which leads from the pump upwards to the center of the tank (which feeds the lower spray arm).
5. Pull the solenoid/valve body off the pipe and you will see it has a whitish plastic flap with a hinge at the top end.
6. Use the flat blade screwdriver to gently prize open one side of the flap hinge and twist the flap until it is completely free of the valve body. Put the flap to one side.
7. Replace the valve body, without the flap, and secure all 4 screws.
8. Reassemble the dishwasher. Done
   

I removed the half wash solenoid on the side of the water pump and notice the plastic flap was ajar and getting forced down with water pressure even though half wash hadnt been selected. i removed the plastic flap, replaced solenoid and bottom spray arm works fine now. we never use the half wash cycle so it was a good result for us.thanks for all your help. regards rob

Just had the exact same fault. Model DW28W.. Removed the flap and presto..all working again.