Detergent Not Dispensing

We have a Whirlpool Duet Front Loading washer, Model GHW9250MW2, and it suddenly is not dispensing the detergent from the dispenser drawer. At the start of the wash cycle, water flows into the softener side of the dispenser drawer, dispensing the softener into the wash cycle, and at no time does the water flow into the detergent side of the drawer to dispense the detergent. How can this problem be corrected?