You need to decalcify it with white vinegar to remove mineral deposits.
Pour 4 cups (20 fl. oz.) of undiluted white vinegar into the water reservoir. Place an empty paper filter in the brew basket, put the empty carafe on the warming plate, and run a brew cycle. Turn it off and let it stand for 30 minutes. Discard the vinegar, rinse the carafe, then run two full cycles (5 cups each) of clean water through the machine to rinse it.
Frequency: Do this every 40 brew cycles for hard water, or every 80 cycles for soft water.