DIAGNOSIS The thermoblock heating system is underperforming. This is usually due to massive scale buildup inside the thermocoil or a failing temperature sensor.
FIX Descale immediately. Use two packs of Breville descaler in sequence. Run the descale cycle twice. Follow with two rinse cycles with fresh water. This dissolves scale that insulates the heater.
IF THAT DIDN'T WORK The NTC temperature sensor might be providing a false reading to the board. This sensor is embedded in the thermoblock and is not user-serviceable. The machine needs service.
HEADS UP Never use vinegar to descale this machine. It can damage the silicone seals in the thermoblock and internal valves. Use citric acid or the branded descaler only.