In some dishwashers there is a rubber plunger/check valve in the back. You don't even need to take the dishwasher out. The part I replaced is called "piston and nut" I only had to replace the plastic piston that has a spring, the rubber seal and the stem. about $10.00.
If that is not it or you have checked the other answer, make sure that your motor is turning all parts and also check the chopper plade and that area for blockage.
Good Luck.
Jerry, June 2006