A flashing display indicates that the heating element is turned off and the kettle is not heating (i.e. the kettle is "idling"). This happens when the hold switch is turned off, your kettle reaches the set target temperature, and sits for 5 minutes OR once you remove it from the base.
If you'd like your kettle to return to the target temperature when you place it back on the base or maintain the target temperature for more than 5 minutes, turn the hold switch on.
With hold switched on, your kettle will maintain the target temperature when you place it on and off the base. With the kettle rested on the base, the heating element will stay engaged for 60 minutes to keep your set temperature. After 60 minutes, the heating element disengages and the kettle will idle for 15 minutes before turning off.
Note: If you notice your kettle doesn't hold the exact temperature and oscillates a few degrees, don't worry. It is likely operating as intended. Rather than having the heating element engaged constantly in hold mode, it pulses on/off and targets the temperature you selected. When it senses the temperature dropping, the heating element kicks in. When the temperature exceeds the target temperature, the heating element shuts off.
So, lets say you set it to 200°F and begin pouring once the kettle reaches temperature. When you place it back on the base and the kettle recognizes the water is now 198°F, the heating element engages to bring it back up to 202°F. When the kettle reaches 202°F, the heating element disengages, but the temperature can continue to creep (up to 203-205°F in this instance).