Bluetooh GATTCCharacteristic Notification

[leo940806]

import machine, ubinascii, utime
from network import Bluetooth
from BleScan import BleScan

def handleNotification(bt_o):
    print('notification handler')

myble = Bluetooth()
con = myble.connect(ble_addr)
srv = con.services()
ch = srv.characteristics()
ch.callback(trigger = Bluetooth.CHAR_NOTIFY_EVENT, handler=handleNotification)

I am using a Wipy to collect data from my BLE sensor. I decided to use the characteristic notification instead of reading the sensor data periodically as the sensing time interval is not regular.
I just extract the essential code as the above.
I am sure the Wipy was connected to my BLE sensor as I was able to read the data manually. However, the Wipy never receive any notification while the BLE sensor was pushing out the data.
Is there any thing I missed for notification configuration? Thank you so much