A Whirlpool dishwasher's detergent dispenser may stay closed during a wash cycle due to old detergent clogging the dispenser door. Another reason may be a burned out trigger mechanism that prevents the door from opening automatically, according to eHow.
Accumulation of old detergent on the dishwasher dispenser door is a common reason for the door remaining closed during a wash cycle. The accumulated detergent does not allow the spring mechanism of the door to function normally, as a result of which the door gets stuck. This problem can be resolved by cleaning the door with hot water and a brush. A combination of hot water and vinegar is also helpful, explains Do It Yourself.
One reason for accumulation of old detergent in the dispenser is a blocked jet in the sprayer, which normally flushes the detergent out during a wash cycle. Cleaning the jets on the sprayer resolves this problem.
Whirlpool dishwasher dispensers have a solenoid or bimetallic trigger mechanism that causes the door of the dispenser to open automatically at an appropriate time in the wash cycle. If this trigger mechanism is burned out, the door cannot open. This problem is fixed by replacing the trigger. Finally, a tall item placed in the dishwasher rack could prevent the dispenser door from opening at the appropriate time.