DIAGNOSIS The detergent dispenser latch mechanism is jammed with hardened detergent or soap scum. Or the solenoid that releases it has failed.
FIX Manually clean the dispenser. Open the door after a cycle when it should have opened. Pour hot water into the dispenser cup to soften any residue. Use a small brush or toothpick to clear the latch area. Work the latch by hand to ensure it moves freely.
IF THAT DIDN'T WORK The dispenser actuator motor (solenoid) is likely faulty. You'll hear no 'click' at the correct time in the cycle. This requires replacing the entire dispenser assembly, which is mounted inside the door.
HEADS UP Using too much detergent or the wrong type can cause this jam. For this model, do not overfill the main cup if using powder or liquid.