I bet you have a jet or two clogged with hard water deposits and/or other crud.
I talked to a dishwasher specialist and he recommends regular Electrasol (now Finish, I guess) powder, not gel, liquid, tablets, or pouches. He says it works better and is better for your dishwasher.
He also told me how to clean my dishwasher so it would work better. And it's amazing the difference it made! It's really simple, too.
With your dishwasher empty, fill a glass with white vinegar and put it right side up on the bottom rack. Run the dishwasher as normal. We do this every time we notice the performance is slipping. When we were having enough problems that we had to call the specialist guy, it took about 4 times before it was really clean.
We also ran the vinegar wash with our dishwasher full, because the dishwasher had been leaving crud all over our dishes that wouldn't wash off by hand. The vinegar cleaned up our dishes 'til they sparkled, too.
I hope that will work for you!