About using ```hcitool``` to get bluetooth rssi

Question

I use sudo hcitool rssi AA:BB:CC:DD:EE:FF to test 2 phones, but the results are 0. I also try to use python program (https://github.com/ewenchou/bluetooth-proximity), the result is also 0. I don't know where went wrong. Besides, I use bluetoothctl-scan on and can get their mac address and rssi although the value is not accuracy. Please, give me some advice.