If your iron isn't steaming properly, turn to the household cure-all: white vinegar. Pour one part vinegar and two parts water into the reservoir. Turn the iron on, and when it's hot, hold it soleplate-side down. (Do not place anything valuable beneath it, because the sediment is going to drip out.) Press the steam button until steam flows freely out the bottom. Pour out the leftover liquid and rinse with distilled water.
A quick trick to conquer a blockage that baffles you.