If it is the first time you use the device, make sure you removed the plastic protection from the sensor, it is not always visible at first glance.
Check that your charger is connected to a power supply with the provided USB cable. Ensure that the bracelet is correctly positioned on the charging station; you should hear a "click" sound and feel the magnetic attraction. If you think a force repulsing the bracelet, try rotating it by 180°.
Try to use a different USB cable or connect to a different USB port (i.e. PC).