1.use a needle to press the networking button of the device,if no light occurs, indicating that battery is dead, please change a new battery with same type.
2.check if the Gateway is connected to network.
3.check if the distance between sensor and gateway is too long.
if the 3 points above can not solve the problem, please take off the battery of the device and power on again, or restore factory settings and add the device to gateway again.