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°. 

If it is the first time you use the device, make sure you removed the plastic protection under the bracelet, it is not always visible at first glance.