BLE beacon scan problem. Never get a result

Question

I debug my BLE scan application.

First I initialize BLE stuff.

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // create the adapter used to populate the list of beacons and attach it to the widget
    scanAdapter = new ScanResultArrayAdapter(this);
    final ListView scanResults = (ListView) findViewById(R.id.listResults);
    scanResults.setAdapter(scanAdapter);

    // set up a handler for taps on the start/stop scanning button
    btn_scan = (Button) findViewById(R.id.buttonScan);
    btn_scan.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View view)
        {
            ToggleScan();
        }
    });

    // retrieve the BluetoothManager instance and check if Bluetooth is enabled. If not the
    // user will be prompted to enable it and the response will be checked in onActivityResult
    final BluetoothManager bluetoothManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
    bleDev = bluetoothManager.getAdapter();
    if (bleDev == null || !bleDev.isEnabled())
    {
        Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
        startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
    }
}

When I press a button I get to the start function ( I omitted ToggleScan it leads to StartScan) . And all goes OK. I see - Starting BLE scan...

private void StartScan()
{
    if(scanner == null)
    {
        scanner = bleDev.getBluetoothLeScanner();
        if(scanner == null)
        {
            // probably tried to start a scan without granting Bluetooth permission
            Toast.makeText(this, "Failed to start scan (BT permission granted?)", Toast.LENGTH_LONG).show();
            Log.w(TAG, "Failed to get BLE scanner instance");
            return;
        }
    }

    Toast.makeText(this, "Starting BLE scan...", Toast.LENGTH_SHORT).show();

    // clear old scan results
    scanAdapter.clear();

    List<ScanFilter> filters = new ArrayList<>();
    ScanSettings settings = new ScanSettings.Builder().setScanMode(scanMode).build();
    scanner.startScan(filters, settings, bleScanCallback);
    isScanning = true;
    btn_scan.setText("Stop scanning");
}

But I never get a callback.

 // class implementing BleScanner callbacks
private ScanCallback bleScanCallback = new ScanCallback()
{

    @Override
    public void onScanResult(int callbackType, ScanResult result)
    {
        super.onScanResult(callbackType, result);

        final BluetoothDevice dev = result.getDevice();
        final int rssi = result.getRssi();

        if(dev != null && isScanning)
        {
            runOnUiThread(new Runnable()
            {
                @Override
                public void run()
                {
                    // retrieve device info and add to or update existing set of beacon data
                    String name = dev.getName();
                    String address = dev.getAddress();
                    scanAdapter.update(dev, address, name == null ? "Unnamed device" : name, rssi);
                }

            });
        }
    }

      //and so on .....

Also I have a device sending beacons. I can see it with a beacons sniffer.

What can be a problem?