Medisana BS 444 - onCharacteristicChanged never triggered

Question

I try for my BLE Scale to write a simple app which displays my weight. I followed the openscale guide for this, but my onCharacteristicChange method never changes. If I pass it manually with nRF Connect (subscribe to the indicate services) and the keyword I get the respective values. I have also tried to implement this with code, but I have no effect with it.

private BluetoothGattCallback serverCallback =
        new BluetoothGattCallback() {
            List<BluetoothGattCharacteristic> characteristics = new ArrayList<>();

            public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
                super.onConnectionStateChange(gatt, status, newState);

                if (newState == BluetoothProfile.STATE_CONNECTED) {
                    Log.d("LOL444", "Connected to GATT.");
                    gatt.discoverServices();
                } else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
                    Log.d("LOL444", "Disconnected from GATT.");
                }


            }

            @Override
            public void onServicesDiscovered(BluetoothGatt gatt, int status) {
                Log.d("LOL444", "onServicesDiscovered");

                if (status == BluetoothGatt.GATT_SUCCESS) {
                    characteristics.add(gatt.getService(WEIGHT_MEASUREMENT_SERVICE)
                            .getCharacteristic(FEATURE_MEASUREMENT_CHARACTERISTIC));
                    characteristics.add(gatt.getService(WEIGHT_MEASUREMENT_SERVICE)
                            .getCharacteristic(WEIGHT_MEASUREMENT_CHARACTERISTIC));
                    characteristics.add(gatt.getService(WEIGHT_MEASUREMENT_SERVICE)
                            .getCharacteristic(CUSTOM5_MEASUREMENT_CHARACTERISTIC));

                    subscribeToCharacteristics(gatt);

                }

            }




            @Override
            public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
                Log.d("LOL444", "onCharacteristicChanged");
                processData(characteristic.getValue(), gatt);
            }

            @Override
            public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
                Log.d("LOL444", "onDescriptorWrite " + status);
                //super.onDescriptorWrite(gatt, descriptor, status);

                if(characteristics.size() > 1){
                    characteristics.remove(0);
                    subscribeToCharacteristics(gatt);
                }
                else{
                    writeCMD(gatt);
                }
            }


            private void subscribeToCharacteristics(BluetoothGatt gatt) {
                Log.d("LOL444", "subscribeToCharacteristics " + characteristics.get(0).getUuid());
                if(characteristics.size() == 0) return;

                BluetoothGattCharacteristic characteristic = characteristics.get(0);
                gatt.setCharacteristicNotification(characteristic, true);

                BluetoothGattDescriptor descriptor = characteristic.getDescriptor(CLIENT_CHARACTERISTIC_CONFIG_UUID);
                if(descriptor != null) {
                    descriptor.setValue(BluetoothGattDescriptor.ENABLE_INDICATION_VALUE);
                    gatt.writeDescriptor(descriptor);
                }
            }


            public void writeCMD(BluetoothGatt gatt){
                Log.d("LOL444", "writeCMD: CMD");

                BluetoothGattCharacteristic characteristic = gatt.getService(WEIGHT_MEASUREMENT_SERVICE)
                        .getCharacteristic(CMD_MEASUREMENT_CHARACTERISTIC);
                
                byte[] magicnumber = {0x02,0x7b,0x7b, (byte) 0xf6,0x0d};
                characteristic.setValue(magicnumber);

            }


        };

I also made sure that the respective write commands are executed one after the other (I think). Thank you in advance and best regards