DIAGNOSIS: The heating element is either not engaging fully or the temperature sensor is miscalibrated, often because of scale buildup if you have hard water.
FIX: Run a descaling cycle. Mix one part white vinegar to three parts water in the external reservoir. Run multiple hot water cycles (using the cup setting, not the carafe) into a sink until the reservoir is empty. Then, fill with fresh water and run several more cycles to flush. This clears mineral deposits off the internal heater and sensor.
IF THAT DIDN'T WORK: The unit might be stuck in a low-power 'energy saving' mode. Go into the settings menu on the screen, find 'Eco Mode' or similar, and disable it. This allows the heater to use full power.
HEADS UP: Never use chemical descaler tablets meant for coffee makers. The vinegar solution is safe for the plastic and seals. If your water is very hard, you might need to descale monthly.