If your tablet is failing to dissolve during the wash cycle you may place the tablet directly on the floor of the machine before closing the door. This bypasses the need to use the dispenser (which purely opens to drop the tablet onto the floor a short time after the cycle starts).
If the dispenser is wet, tablets can stick to the inside and will not drop out and away onto the dishwasher floor. Using the method above bypasses the need for the dispenser.