QBluetoothDeviceDiscoveryAgent class start() method invalid

I Set Discovery timeout 5s,when finished restart Discovery ,but a long time about one hour,can not search any BLE device?

like this:

m_deviceDiscoveryAgentBLE = new QBluetoothDeviceDiscoveryAgent( this );
m_deviceDiscoveryAgentBLE->setLowEnergyDiscoveryTimeout( 5000 );
connect( m_deviceDiscoveryAgentBLE, &QBluetoothDeviceDiscoveryAgent::finished, this, &BLEDeviceFinder::scanFinished );

void BLEDeviceFinder::scanFinished()

{           m_deviceDiscoveryAgentBLE->start( QBluetoothDeviceDiscoveryAgent::LowEnergyMethod ); }