I was having a similar problem with my Whirlpool dishwasher not cleaning well anymore, especially the top rack. I opened it up a few times, and it didn't look like the spray arm had moved at all. It didn't look clogged, however. After following this post, I looked to see the water level, and it didn't come up to the heating element. So I took the fill valve off and disassembled it:
(Sorry - I forgot to resize the image)
The screen was full of sediment! I couldn't get the screen out without damaging it, and I didn't have another handy, so I wound up cleaning it with a Water Pic and a toothbrush. After reassembly (careful not the strip the screw threads!), and reinstallation, the dishwasher cleans like new. Thanks for all the advice.