The USB cable may be faulty or connected to a source that is not able to provide the charge level required.
Plug the USB charging cable into a different USB port on your PC or plug the USB charging cable into a wall adaptor capable of at least 500mA. Alternatively, you can try another standard micro-USB charging cable. If none of the above solves your issue, please contact the Customer Care Team. Always try to use the USB charging cable supplied. You can buy a replacement USB charging cable on www.slendertone.com.